[clang] Fix crash upon stray coloncolon token in C2x mode

The parser assumes that the lexer never emits coloncolon token for C code, but this assumption no longer holds in C2x attribute namespaces. As a result, stray coloncolon tokens out of attributes cause assertion failures and hangs in release build, which this patch tries to handle.

Crash input minimal example: `T n::v`

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D133248