[clang] Deprecate DirectoryEntry::getName()

This finally officially deprecates `DirectoryEntry::getName()`. I checked no usages remain in targets built by any of `check-clang`, `check-clang-tools`, `check-clang-extra`. There are probably some remaining usages in places like LLDB and other clients. This will give them a chance to transition to `DirectoryEntryRef::getName()` before we remove the function altogether.

Depends on D151922.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D151927

clang/include/clang/Basic/DirectoryEntry.h

@@ -46,6 +46,7 @@ class DirectoryEntry {
   StringRef Name; // Name of the directory.
 
 public:
+  LLVM_DEPRECATED("Use DirectoryEntryRef::getName() instead.", "")
   StringRef getName() const { return Name; }
 };