[pseudo] Implement guard extension.

- Extend the GLR parser to allow conditional reduction based on the
  guard functions;
- Implement two simple guards (contextual-override/final) for cxx.bnf;
- layering: clangPseudoCXX depends on clangPseudo (as the guard function need
  to access the TokenStream);

Differential Revision: https://reviews.llvm.org/D127448