[Frontend] 'Show hotness' can be used with a sampling profile

Summary:
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.

Reviewers: anemet, davidxl

Reviewed By: davidxl

Subscribers: danielcdh, cfe-commits

Differential Revision: https://reviews.llvm.org/D34082

llvm-svn: 306079