[ADT] IntervalMap: add overlaps(a, b) method
This function checks whether the mappings in the interval map overlap
with the given range [a;b]. The motivation is to enable checking for
overlap before inserting a new interval into the map.
Reviewers: vsk, dblaikie
Subscribers: dexonsmith, kristina, llvm-commits
Differential Revision: https://reviews.llvm.org/D55760