Prior to this change, using -fdiagnostics-show-hotness with a sampling
profile specified via -fprofile-sample-use= would result in the Clang
frontend emitting a warning: "argument '-fdiagnostics-show-hotness' requires
profile-guided optimization information". Of course, a sampling profile
*is* profile-guided optimization information, so the warning is misleading.
Furthermore, despite the warning, hotness was displayed based on the data in
the sampling profile.
Prevent the warning from being emitted when a sampling profile is used, and
add a test that verifies this.