[clang] Add error check for HeuristicResolver (PR #155561)

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 approved this pull request.

LGTM, thanks!

Since the patch is making some minor changes to libTooling, I'm going to wait a couple of days before merging, in case anyone else has comments.

https://github.com/llvm/llvm-project/pull/155561