[analyzer] Implemented RangeSet::Factory::unite function to handle intersections and adjacency

Summary: Handle intersected and adjacent ranges uniting them into a single one.
Example:
intersection [0, 10] U [5, 20] = [0, 20]
adjacency [0, 10] U [11, 20] = [0, 20]

Differential Revision: https://reviews.llvm.org/D99797