[pseudo] Simplify the glrReduce implementation.

glrReduce maintains two priority queues (one for bases, and the other
for Sequence), these queues are in parallel with each other, corresponding to a
single family. They can be folded into one.

This patch removes the bases priority queue, which improves the glrParse by
10%.

ASTReader.cpp: 2.03M/s (before) vs 2.26M/s (after)

Differential Revision: https://reviews.llvm.org/D127283