[Driver] Add -fdiagnostics-hotness-threshold

Summary:
Depends on https://reviews.llvm.org/D34867.

Add a Clang frontend option to enable optimization remark hotness
thresholds, which were added to LLVM in https://reviews.llvm.org/D34867.
This prevents diagnostics that do not meet a minimum hotness
threshold from being output. When generating optimization remarks for large
codebases with a ton of cold code paths, this option can be used
to limit the optimization remark output at a reasonable size.
Discussion of this change can be read here:
http://lists.llvm.org/pipermail/llvm-dev/2017-June/114377.html

Reviewers: anemet, davidxl, hfinkel

Reviewed By: anemet

Subscribers: fhahn, cfe-commits

Differential Revision: https://reviews.llvm.org/D34868

llvm-svn: 306945