[ADT] IntervalTree - Fix random unittests failures in a debug builds.

On a debug build with _LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY
enabled from 100 executions around 80 are failing.

More details in https://reviews.llvm.org/D125776#3820399

The issue is related to the use of std::sort.

Reviewed By: antondaubert, jryans, probinson

Differential Revision: https://reviews.llvm.org/D134805