[pseudo] A basic implementation of compiling cxx grammar at build time.

[pseudo] A basic implementation of compiling cxx grammar at build time.

The main idea is to compile the cxx grammar at build time, and construct
the core pieces (Grammar, LRTable) of the pseudoparse based on the compiled
data sources.

This is a tiny implementation, which is good for start:

- defines how the public API should look like;
- integrates the cxx grammar compilation workflow with the cmake system.
- onlynonterminal symbols of the C++ grammar are compiled, anything
  else are still doing the real compilation work at runtime, we can opt-in more
  bits in the future;
- splits the monolithic clangPsuedo library for better layering;

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D125667