[clang] [clang] [NFC] explicitly check if ParentMap contains key (PR #121736)

[Jay Foad via cfe-commits]

https://github.com/jayfoad commented:

No objections, just a couple of ideas for improvements. I have no idea if `ParentMap` lookups are on any kind of hot path.

https://github.com/llvm/llvm-project/pull/121736