[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