[Remarks] Make sure -fdiagnostics-hotness-threshold implies -fdiagnostics-show-hotness

When asking for -fdiagnostics-hotness-threshold, we currently require
-fdiagnostics-show-hotness otherwise we silently display nothing.

I don't see a scenario where that makes sense, so have one option impy
the other.

Differential Revision: https://reviews.llvm.org/D153746