[clang-tidy] Fix RenamerClangTidy checks trying to emit a fix that isnt a valid identifier

Addresses https://bugs.llvm.org/show_bug.cgi?id=48230.
Handle the case when the Fixup suggested isn't a valid c/c++ identifer.

Reviewed By: aaron.ballman, gribozavr2

Differential Revision: https://reviews.llvm.org/D91915