[clang][Lex] Add back PPCallbacks::FileNotFound

This callback was removed with commit 7a124f48, but we use it
downstream in ROOT/Cling to implement handling of a special include
syntax. Add back a "safe" version of the callback that only takes
the file name and return a bool to silently skip the file.

Differential Revision: https://reviews.llvm.org/D142196