Frontend: Migrate to FileEntryRef in CompilerInstance::InitializeSourceManager, NFC

[dexonsmith]

Summary

Use FileManager::getVirtualFileRef to get the virtual file for stdin,
and add an overload of SourceManager::overrideFileContents that takes
a FileEntryRef, migrating CompilerInstance::InitializeSourceManager.

[jansvoboda11]

LGTM.

clang/include/clang/Basic/SourceManager.h

public:
   /// already-allocated buffer.
   ///
   /// \param SourceFile the source file whose contents will be overridden.
   ///
   /// \param Buffer the memory buffer whose contents will be used as the
   /// data in the given source file.
   void overrideFileContents(const FileEntry *SourceFile,
                             std::unique_ptr<llvm::MemoryBuffer> Buffer);
+  void overrideFileContents(FileEntryRef SourceFile,
+                            std::unique_ptr<llvm::MemoryBuffer> Buffer) {
+    overrideFileContents(&SourceFile.getFileEntry(), std::move(Buffer));
+  }
 
   /// Override the given source file with another one.
   ///
   /// \param SourceFile the source file which will be overridden.
   ///
   /// \param NewFile the file whose contents will be used as the
   /// data instead of the contents of the given source file.
   void overrideFileContents(const FileEntry *SourceFile,

clang/lib/Frontend/CompilerInstance.cpp

if (InputFile != "-") {
     llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> SBOrErr =
         llvm::MemoryBuffer::getSTDIN();
     if (std::error_code EC = SBOrErr.getError()) {
       Diags.Report(diag::err_fe_error_reading_stdin) << EC.message();
       return false;
     }
     std::unique_ptr<llvm::MemoryBuffer> SB = std::move(SBOrErr.get());
 
-    const FileEntry *File = FileMgr.getVirtualFile(SB->getBufferIdentifier(),
-                                                   SB->getBufferSize(), 0);
+    FileEntryRef File = FileMgr.getVirtualFileRef(SB->getBufferIdentifier(),
     SourceMgr.setMainFileID(
         SourceMgr.createFileID(File, SourceLocation(), Kind));
     SourceMgr.overrideFileContents(File, std::move(SB));
   }
 
   assert(SourceMgr.getMainFileID().isValid() &&
          "Couldn't establish MainFileID!");
   return true;