[clang] NFCI: Use FileEntryRef in ASTWriter

Author: jansvoboda11

clang/lib/Serialization/ASTWriter.cpp

@@ -1550,7 +1550,7 @@ void ASTWriter::WriteControlBlock(Preprocessor &PP, ASTContext &Context,
 
   // Original file name and file ID
   SourceManager &SM = Context.getSourceManager();
-  if (const FileEntry *MainFile = SM.getFileEntryForID(SM.getMainFileID())) {
+  if (auto MainFile = SM.getFileEntryRefForID(SM.getMainFileID())) {
     auto FileAbbrev = std::make_shared<BitCodeAbbrev>();
     FileAbbrev->Add(BitCodeAbbrevOp(ORIGINAL_FILE));
     FileAbbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // File ID