[MemProf] Add hot allocation type

Add "Hot" AllocationType (in addition to existing cold, notcold).

Use lifetime access density as metric to identify hot allocations.
Treat hot as notcold for MemProfContextDisambiguation for now
before the disambiguation for "hot" is done.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D149932