Index: lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
===================================================================
--- lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
+++ lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
@@ -9,6 +9,7 @@
 #ifndef LLDB_SOURCE_PLUGINS_EXPRESSIONPARSER_CLANG_ASTUTILS_H
 #define LLDB_SOURCE_PLUGINS_EXPRESSIONPARSER_CLANG_ASTUTILS_H
 
+#include "Plugins/TypeSystem/Clang/ImporterBackedASTSource.h"
 #include "clang/Basic/Module.h"
 #include "clang/Sema/Lookup.h"
 #include "clang/Sema/MultiplexExternalSemaSource.h"
@@ -19,7 +20,7 @@
 
 /// Wraps an ExternalASTSource into an ExternalSemaSource. Doesn't take
 /// ownership of the provided source.
-class ExternalASTSourceWrapper : public clang::ExternalSemaSource {
+class ExternalASTSourceWrapper : public ImporterBackedASTSource {
   ExternalASTSource *m_Source;
 
 public:
@@ -239,7 +240,7 @@
 /// provide more accurate replies to the requests, but might not be able to
 /// answer all requests. The debug information will be used as a fallback then
 /// to provide information that is not in the C++ module.
-class SemaSourceWithPriorities : public clang::ExternalSemaSource {
+class SemaSourceWithPriorities : public ImporterBackedASTSource {
 
 private:
   /// The sources ordered in decreasing priority.
Index: lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.h
===================================================================
--- lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.h
+++ lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.h
@@ -13,6 +13,7 @@
 
 #include "Plugins/ExpressionParser/Clang/ClangASTImporter.h"
 #include "Plugins/ExpressionParser/Clang/NameSearchContext.h"
+#include "Plugins/TypeSystem/Clang/ImporterBackedASTSource.h"
 #include "lldb/Symbol/CompilerType.h"
 #include "lldb/Target/Target.h"
 #include "clang/AST/ExternalASTSource.h"
@@ -30,7 +31,7 @@
 /// knows the name it is looking for, but nothing else. The ExternalSemaSource
 /// class provides Decls (VarDecl, FunDecl, TypeDecl) to Clang for these
 /// names, consulting the ClangExpressionDeclMap to do the actual lookups.
-class ClangASTSource : public clang::ExternalASTSource,
+class ClangASTSource : public ImporterBackedASTSource,
                        public ClangASTImporter::MapCompleter {
 public:
   /// Constructor
Index: lldb/source/Plugins/ExpressionParser/Clang/ClangExternalASTSourceCallbacks.h
===================================================================
--- lldb/source/Plugins/ExpressionParser/Clang/ClangExternalASTSourceCallbacks.h
+++ lldb/source/Plugins/ExpressionParser/Clang/ClangExternalASTSourceCallbacks.h
@@ -9,18 +9,21 @@
 #ifndef LLDB_SOURCE_PLUGINS_EXPRESSIONPARSER_CLANG_CLANGEXTERNALASTSOURCECALLBACKS_H
 #define LLDB_SOURCE_PLUGINS_EXPRESSIONPARSER_CLANG_CLANGEXTERNALASTSOURCECALLBACKS_H
 
+#include "Plugins/TypeSystem/Clang/ImporterBackedASTSource.h"
 #include "Plugins/TypeSystem/Clang/TypeSystemClang.h"
 #include "clang/Basic/Module.h"
 
 namespace lldb_private {
 
-class ClangExternalASTSourceCallbacks : public clang::ExternalASTSource {
+class ClangExternalASTSourceCallbacks : public ImporterBackedASTSource {
   /// LLVM RTTI support.
   static char ID;
 
 public:
   /// LLVM RTTI support.
-  bool isA(const void *ClassID) const override { return ClassID == &ID; }
+  bool isA(const void *ClassID) const override {
+    return ClassID == &ID || ImporterBackedASTSource::isA(ClassID);
+  }
   static bool classof(const clang::ExternalASTSource *s) { return s->isA(&ID); }
 
   ClangExternalASTSourceCallbacks(TypeSystemClang &ast) : m_ast(ast) {}
Index: lldb/source/Plugins/TypeSystem/Clang/CMakeLists.txt
===================================================================
--- lldb/source/Plugins/TypeSystem/Clang/CMakeLists.txt
+++ lldb/source/Plugins/TypeSystem/Clang/CMakeLists.txt
@@ -1,4 +1,5 @@
 add_lldb_library(lldbPluginTypeSystemClang PLUGIN
+  ImporterBackedASTSource.cpp
   TypeSystemClang.cpp
 
   LINK_LIBS
Index: lldb/source/Plugins/TypeSystem/Clang/ImporterBackedASTSource.h
===================================================================
--- /dev/null
+++ lldb/source/Plugins/TypeSystem/Clang/ImporterBackedASTSource.h
@@ -0,0 +1,46 @@
+//===-- ImporterBackedASTSource.h -------------------------------*- 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 LLDB_SOURCE_PLUGINS_TYPESYSTEM_CLANG_IMPORTERBACKEDASTSOURCE
+#define LLDB_SOURCE_PLUGINS_TYPESYSTEM_CLANG_IMPORTERBACKEDASTSOURCE
+
+#include "clang/AST/ASTContext.h"
+#include "clang/Sema/ExternalSemaSource.h"
+
+namespace lldb_private {
+
+/// The base class of all ExternalASTSources in LLDB that use the
+/// ClangASTImporter to move declarations from other ASTs to the ASTContext they
+/// are attached to.
+class ImporterBackedASTSource : public clang::ExternalSemaSource {
+  /// LLVM RTTI support.
+  static char ID;
+
+public:
+  /// LLVM RTTI support.
+  bool isA(const void *ClassID) const override {
+    return ClassID == &ID || ExternalSemaSource::isA(ClassID);
+  }
+  static bool classof(const clang::ExternalASTSource *s) { return s->isA(&ID); }
+
+  /// This marks all redeclaration chains in the ASTContext as out-of-date and
+  /// that this ExternalASTSource should be consulted to get the complete
+  /// redeclaration chain.
+  ///
+  /// \see ExternalASTSource::CompleteRedeclChain
+  void MarkRedeclChainsAsOutOfDate(clang::ASTContext &c) {
+    // This invalidates redeclaration chains but also other things such as
+    // identifiers. There isn't a more precise way at the moment that only
+    // affects redecl chains.
+    incrementGeneration(c);
+  }
+};
+
+} // namespace lldb_private
+
+#endif // LLDB_SOURCE_PLUGINS_TYPESYSTEM_CLANG_IMPORTERBACKEDASTSOURCE
Index: lldb/source/Plugins/TypeSystem/Clang/ImporterBackedASTSource.cpp
===================================================================
--- /dev/null
+++ lldb/source/Plugins/TypeSystem/Clang/ImporterBackedASTSource.cpp
@@ -0,0 +1,13 @@
+//===-- ImporterBackedASTSource.cpp ---------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "Plugins/TypeSystem/Clang/ImporterBackedASTSource.h"
+
+using namespace lldb_private;
+
+char ImporterBackedASTSource::ID;