Index: clang/include/clang/Basic/SourceManager.h
===================================================================
--- clang/include/clang/Basic/SourceManager.h
+++ clang/include/clang/Basic/SourceManager.h
@@ -35,6 +35,7 @@
 #define LLVM_CLANG_BASIC_SOURCEMANAGER_H
 
 #include "clang/Basic/Diagnostic.h"
+#include "clang/Basic/FileManager.h"
 #include "clang/Basic/SourceLocation.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/BitVector.h"
@@ -59,9 +60,6 @@
 
 class ASTReader;
 class ASTWriter;
-class FileManager;
-class FileEntry;
-class FileEntryRef;
 class LineTableInfo;
 class SourceManager;
 
@@ -706,6 +704,9 @@
   /// The file ID for the main source file of the translation unit.
   FileID MainFileID;
 
+  /// The file entry for the main source file.
+  Optional<Optional<FileEntryRef>> CachedMainFileEntry;
+
   /// The file ID for the precompiled preamble there is one.
   FileID PreambleFileID;
 
@@ -813,6 +814,9 @@
     MainFileID = FID;
   }
 
+  /// Returns true when the given FileEntry corresponds to the main file.
+  bool isMainFile(FileEntryRef SourceFile);
+
   /// Set the file ID for the precompiled preamble.
   void setPreambleFileID(FileID Preamble) {
     assert(PreambleFileID.isInvalid() && "PreambleFileID already set!");
Index: clang/lib/Basic/SourceManager.cpp
===================================================================
--- clang/lib/Basic/SourceManager.cpp
+++ clang/lib/Basic/SourceManager.cpp
@@ -373,6 +373,7 @@
 
 void SourceManager::clearIDTables() {
   MainFileID = FileID();
+  CachedMainFileEntry = Optional<Optional<FileEntryRef>>();
   LocalSLocEntryTable.clear();
   LoadedSLocEntryTable.clear();
   SLocEntryLoaded.clear();
@@ -389,6 +390,15 @@
   createExpansionLoc(SourceLocation(), SourceLocation(), SourceLocation(), 1);
 }
 
+bool SourceManager::isMainFile(FileEntryRef SourceFile) {
+  assert(MainFileID.isValid() && "expected initialized SourceManager");
+  if (!CachedMainFileEntry)
+    *CachedMainFileEntry = getFileEntryRefForID(MainFileID);
+  if (!*CachedMainFileEntry)
+    return false;
+  return (*CachedMainFileEntry)->getUID() == SourceFile.getUID();
+}
+
 void SourceManager::initializeForReplay(const SourceManager &Old) {
   assert(MainFileID.isInvalid() && "expected uninitialized SourceManager");
 
Index: clang/lib/Lex/PPDirectives.cpp
===================================================================
--- clang/lib/Lex/PPDirectives.cpp
+++ clang/lib/Lex/PPDirectives.cpp
@@ -2054,8 +2054,7 @@
   // some directives (e.g. #endif of a header guard) will never be seen.
   // Since this will lead to confusing errors, avoid the inclusion.
   if (Action == Enter && File && PreambleConditionalStack.isRecording() &&
-      SourceMgr.translateFile(&File->getFileEntry()) ==
-          SourceMgr.getMainFileID()) {
+      SourceMgr.isMainFile(*File)) {
     Diag(FilenameTok.getLocation(),
          diag::err_pp_including_mainfile_in_preamble);
     return {ImportAction::None};