clang-format: Migrate createInMemoryFile to FileEntryRef, NFC

GitOrigin-RevId: 75a95bc80eae0c539b342f2cb19a92a0de02d2e1
diff --git a/ClangFormat.cpp b/ClangFormat.cpp
index 121c962..a1b42a6 100644
--- a/ClangFormat.cpp
+++ b/ClangFormat.cpp
@@ -178,7 +178,7 @@
                                  SourceManager &Sources, FileManager &Files,
                                  llvm::vfs::InMemoryFileSystem *MemFS) {
   MemFS->addFileNoOwn(FileName, 0, Source);
-  auto File = Files.getFile(FileName);
+  auto File = Files.getOptionalFileRef(FileName);
   assert(File && "File not added to MemFS?");
   return Sources.createFileID(*File, SourceLocation(), SrcMgr::C_User);
 }