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

Fix build failure in:

  https://lab.llvm.org/buildbot/#/builders/36/builds/25424
  error: comparison of integers of different signs:
  'const unsigned long' and 'const int' [-Werror,-Wsign-compare]

  Reviewed By: Orlando

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