[clang] NFCI: Use DirectoryEntryRef in FrontendAction::BeginSourceFile()

This patch removes use of the deprecated `DirectoryEntry::getName()` from `FrontendAction::BeginSourceFile()`.

Reviewed By: bnbarham

Differential Revision: https://reviews.llvm.org/D123853

Author: jansvoboda11

clang/lib/Frontend/FrontendAction.cpp

@@ -761,10 +761,10 @@ bool FrontendAction::BeginSourceFile(CompilerInstance &CI,
     PreprocessorOptions &PPOpts = CI.getPreprocessorOpts();
     StringRef PCHInclude = PPOpts.ImplicitPCHInclude;
     std::string SpecificModuleCachePath = CI.getSpecificModuleCachePath();
-    if (auto PCHDir = FileMgr.getDirectory(PCHInclude)) {
+    if (auto PCHDir = FileMgr.getOptionalDirectoryRef(PCHInclude)) {
       std::error_code EC;
       SmallString<128> DirNative;
-      llvm::sys::path::native((*PCHDir)->getName(), DirNative);
+      llvm::sys::path::native(PCHDir->getName(), DirNative);
       bool Found = false;
       llvm::vfs::FileSystem &FS = FileMgr.getVirtualFileSystem();
       for (llvm::vfs::directory_iterator Dir = FS.dir_begin(DirNative, EC),