[clang] [Lex][Clang] Add checking to HeaderMapImpl::getString to make it more robust (PR #131677)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM with nit addressed. Test coverage would be nice to add, but not strictly required if this is purely a defensive measure (which it seems to be).

https://github.com/llvm/llvm-project/pull/131677