[pseudo] Implement LRTable.

This patch introduces a dense implementation of the LR parsing table, which is
used by LR parsers.

We build a SLR(1) parsing table from the LR(0) graph.

Statistics of the LR parsing table on the C++ spec grammar:
  - number of states: 1449
  - number of actions: 83069
  - size of the table (bytes): 334928

Differential Revision: https://reviews.llvm.org/D118196