[pseudo] Implement the GLR parsing algorithm.

This patch implements a standard GLR parsing algorithm, the
core piece of the pseudoparser.

- it parses preprocessed C++ code, currently it supports correct code
  only and parse them as a translation-unit;
- it produces a forest which stores all possible trees in an efficient
  manner (only a single node being build for per (SymbolID, Token Range));
  no disambiguation yet;

Reland with a fix for g++'s -fpermissive error on previous declaration `GSS& GSS;`.

Differential Revision: https://reviews.llvm.org/D121150