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