diff --git a/clang/lib/APINotes/APINotesFormat.h b/clang/lib/APINotes/APINotesFormat.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/APINotes/APINotesFormat.h
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+//===-- APINotesWriter.h - API Notes Writer ---------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_LIB_APINOTES_APINOTESFORMAT_H
+#define LLVM_CLANG_LIB_APINOTES_APINOTESFORMAT_H
+
+#include "llvm/ADT/PointerEmbeddedInt.h"
+#include "llvm/Bitcode/BitcodeConvenience.h"
+
+namespace clang {
+namespace api_notes {
+/// Magic number for API notes files.
+const unsigned char API_NOTES_SIGNATURE[] = {0xE2, 0x9C, 0xA8, 0x01};
+
+/// API notes file major version number.
+const uint16_t VERSION_MAJOR = 0;
+
+/// API notes file minor version number.
+///
+/// When the format changes IN ANY WAY, this number should be incremented.
+const uint16_t VERSION_MINOR = 24; // EnumExtensibility + FlagEnum
+
+using IdentifierID = llvm::PointerEmbeddedInt<unsigned, 31>;
+using IdentifierIDField = llvm::BCVBR<16>;
+
+using SelectorID = llvm::PointerEmbeddedInt<unsigned, 31>;
+using SelectorIDField = llvm::BCVBR<16>;
+
+/// The various types of blocks that can occur within a API notes file.
+///
+/// These IDs must \em not be renumbered or reordered without incrementing
+/// VERSION_MAJOR.
+enum BlockID {
+  /// The control block, which contains all of the information that needs to
+  /// be validated prior to committing to loading the API notes file.
+  ///
+  /// \sa control_block
+  CONTROL_BLOCK_ID = llvm::bitc::FIRST_APPLICATION_BLOCKID,
+
+  /// The identifier data block, which maps identifier strings to IDs.
+  IDENTIFIER_BLOCK_ID,
+
+  /// The Objective-C context data block, which contains information about
+  /// Objective-C classes and protocols.
+  OBJC_CONTEXT_BLOCK_ID,
+
+  /// The Objective-C property data block, which maps Objective-C
+  /// (class name, property name) pairs to information about the
+  /// property.
+  OBJC_PROPERTY_BLOCK_ID,
+
+  /// The Objective-C property data block, which maps Objective-C
+  /// (class name, selector, is_instance_method) tuples to information
+  /// about the method.
+  OBJC_METHOD_BLOCK_ID,
+
+  /// The Objective-C selector data block, which maps Objective-C
+  /// selector names (# of pieces, identifier IDs) to the selector ID
+  /// used in other tables.
+  OBJC_SELECTOR_BLOCK_ID,
+
+  /// The global variables data block, which maps global variable names to
+  /// information about the global variable.
+  GLOBAL_VARIABLE_BLOCK_ID,
+
+  /// The (global) functions data block, which maps global function names to
+  /// information about the global function.
+  GLOBAL_FUNCTION_BLOCK_ID,
+
+  /// The tag data block, which maps tag names to information about
+  /// the tags.
+  TAG_BLOCK_ID,
+
+  /// The typedef data block, which maps typedef names to information about
+  /// the typedefs.
+  TYPEDEF_BLOCK_ID,
+
+  /// The enum constant data block, which maps enumerator names to
+  /// information about the enumerators.
+  ENUM_CONSTANT_BLOCK_ID,
+};
+
+namespace control_block {
+// These IDs must \em not be renumbered or reordered without incrementing
+// VERSION_MAJOR.
+enum {
+  METADATA = 1,
+  MODULE_NAME = 2,
+  MODULE_OPTIONS = 3,
+  SOURCE_FILE = 4,
+};
+
+using MetadataLayout =
+    llvm::BCRecordLayout<METADATA,          // ID
+                         llvm::BCFixed<16>, // Module format major version
+                         llvm::BCFixed<16>  // Module format minor version
+                         >;
+
+using ModuleNameLayout = llvm::BCRecordLayout<MODULE_NAME,
+                                              llvm::BCBlob // Module name
+                                              >;
+
+using ModuleOptionsLayout =
+    llvm::BCRecordLayout<MODULE_OPTIONS,
+                         llvm::BCFixed<1> // SwiftInferImportAsMember
+                         >;
+
+using SourceFileLayout = llvm::BCRecordLayout<SOURCE_FILE,
+                                              llvm::BCVBR<16>, // file size
+                                              llvm::BCVBR<16>  // creation time
+                                              >;
+} // namespace control_block
+
+namespace identifier_block {
+enum {
+  IDENTIFIER_DATA = 1,
+};
+
+using IdentifierDataLayout = llvm::BCRecordLayout<
+    IDENTIFIER_DATA, // record ID
+    llvm::BCVBR<16>, // table offset within the blob (see below)
+    llvm::BCBlob     // map from identifier strings to decl kinds / decl IDs
+    >;
+} // namespace identifier_block
+
+namespace objc_context_block {
+enum {
+  OBJC_CONTEXT_ID_DATA = 1,
+  OBJC_CONTEXT_INFO_DATA = 2,
+};
+
+using ObjCContextIDLayout =
+    llvm::BCRecordLayout<OBJC_CONTEXT_ID_DATA, // record ID
+                         llvm::BCVBR<16>, // table offset within the blob (see
+                                          // below)
+                         llvm::BCBlob // map from ObjC class names/protocol (as
+                                      // IDs) to context IDs
+                         >;
+
+using ObjCContextInfoLayout = llvm::BCRecordLayout<
+    OBJC_CONTEXT_INFO_DATA, // record ID
+    llvm::BCVBR<16>,        // table offset within the blob (see below)
+    llvm::BCBlob            // map from ObjC context IDs to context information.
+    >;
+} // namespace objc_context_block
+
+namespace objc_property_block {
+enum {
+  OBJC_PROPERTY_DATA = 1,
+};
+
+using ObjCPropertyDataLayout = llvm::BCRecordLayout<
+    OBJC_PROPERTY_DATA, // record ID
+    llvm::BCVBR<16>,    // table offset within the blob (see below)
+    llvm::BCBlob        // map from ObjC (class name, property name) pairs to
+                        // ObjC property information
+    >;
+} // namespace objc_property_block
+
+namespace objc_method_block {
+enum {
+  OBJC_METHOD_DATA = 1,
+};
+
+using ObjCMethodDataLayout =
+    llvm::BCRecordLayout<OBJC_METHOD_DATA, // record ID
+                         llvm::BCVBR<16>,  // table offset within the blob (see
+                                           // below)
+                         llvm::BCBlob // map from ObjC (class names, selector,
+                                      // is-instance-method) tuples to ObjC
+                                      // method information
+                         >;
+} // namespace objc_method_block
+
+namespace objc_selector_block {
+enum {
+  OBJC_SELECTOR_DATA = 1,
+};
+
+using ObjCSelectorDataLayout =
+    llvm::BCRecordLayout<OBJC_SELECTOR_DATA, // record ID
+                         llvm::BCVBR<16>, // table offset within the blob (see
+                                          // below)
+                         llvm::BCBlob // map from (# pieces, identifier IDs) to
+                                      // Objective-C selector ID.
+                         >;
+} // namespace objc_selector_block
+
+namespace global_variable_block {
+enum { GLOBAL_VARIABLE_DATA = 1 };
+
+using GlobalVariableDataLayout = llvm::BCRecordLayout<
+    GLOBAL_VARIABLE_DATA, // record ID
+    llvm::BCVBR<16>,      // table offset within the blob (see below)
+    llvm::BCBlob          // map from name to global variable information
+    >;
+} // namespace global_variable_block
+
+namespace global_function_block {
+enum { GLOBAL_FUNCTION_DATA = 1 };
+
+using GlobalFunctionDataLayout = llvm::BCRecordLayout<
+    GLOBAL_FUNCTION_DATA, // record ID
+    llvm::BCVBR<16>,      // table offset within the blob (see below)
+    llvm::BCBlob          // map from name to global function information
+    >;
+} // namespace global_function_block
+
+namespace tag_block {
+enum { TAG_DATA = 1 };
+
+using TagDataLayout =
+    llvm::BCRecordLayout<TAG_DATA,        // record ID
+                         llvm::BCVBR<16>, // table offset within the blob (see
+                                          // below)
+                         llvm::BCBlob     // map from name to tag information
+                         >;
+}; // namespace tag_block
+
+namespace typedef_block {
+enum { TYPEDEF_DATA = 1 };
+
+using TypedefDataLayout =
+    llvm::BCRecordLayout<TYPEDEF_DATA,    // record ID
+                         llvm::BCVBR<16>, // table offset within the blob (see
+                                          // below)
+                         llvm::BCBlob // map from name to typedef information
+                         >;
+}; // namespace typedef_block
+
+namespace enum_constant_block {
+enum { ENUM_CONSTANT_DATA = 1 };
+
+using EnumConstantDataLayout =
+    llvm::BCRecordLayout<ENUM_CONSTANT_DATA, // record ID
+                         llvm::BCVBR<16>, // table offset within the blob (see
+                                          // below)
+                         llvm::BCBlob // map from name to enumerator information
+                         >;
+} // namespace enum_constant_block
+
+/// A stored Objective-C selector.
+struct StoredObjCSelector {
+  unsigned NumPieces;
+  llvm::SmallVector<IdentifierID, 2> Identifiers;
+};
+} // namespace api_notes
+} // namespace clang
+
+#endif