[MemProf] Optionally pass hot/cold hints to operator new

Optionally (off by default) replace operator new() calls marked with a
hot or cold memprof attribute with an operator new() call that takes a
hot_cold_t parameter.

Currently this is supported by the open source version of tcmalloc, see:
https://github.com/google/tcmalloc/blob/master/tcmalloc/new_extension.h

Differential Revision: https://reviews.llvm.org/D148718