[syntax][pseudo] Add Grammar for the clang pseudo-parser

This patch introduces the Grammar class, which is a critial piece for constructing
a tabled-based parser.

As the first patch, the scope is limited to:
  - define base types (symbol, rules) of modeling the grammar
  - construct Grammar by parsing the BNF file (annotations are excluded for now)

Differential Revision: https://reviews.llvm.org/D114790