[ADT] Add IntervalTree - light tree data structure to hold intervals.

It allows finding all intervals that overlap with any given point.
At this time, it does not support any deletion or rebalancing
operations.

The IntervalTree is designed to be set up once, and then queried
without any further additions.

Reviewed By: psamolysov, probinson

Differential Revision: https://reviews.llvm.org/D125776