Index: clang/include/clang/Basic/SourceManager.h
===================================================================
--- clang/include/clang/Basic/SourceManager.h
+++ clang/include/clang/Basic/SourceManager.h
@@ -813,6 +813,9 @@
     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
@@ -389,6 +389,14 @@
   createExpansionLoc(SourceLocation(), SourceLocation(), SourceLocation(), 1);
 }
 
+bool SourceManager::isMainFile(FileEntryRef SourceFile) {
+  assert(MainFileID.isValid() && "expected initialized SourceManager");
+  auto FE = getFileEntryRefForID(MainFileID);
+  if (!FE)
+    return false;
+  return FE->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};