[clang][lex] Remove `PPCallbacks::FileNotFound()`

The purpose of the `FileNotFound` preprocessor callback was to add the ability to recover from failed header lookups. This was to support downstream project.

However, injecting additional search path while performing header search can invalidate currently used iterators/references to `DirectoryLookup` in `Preprocessor` and `HeaderSearch`.

The downstream project ended up maintaining a separate patch to further tweak the functionality. Since we don't have any upstream users nor open source downstream users, I'd like to remove this callback for good to prevent future misuse. I doubt there are any actual downstream users, since the functionality is definitely broken at the moment.

Reviewed By: ahoppen

Differential Revision: https://reviews.llvm.org/D119708