[ORE] Use LLVM's "diagnostics hotness" spelling

Summary:
Depends on https://reviews.llvm.org/D34864.

To unify Clang and LLVM's spelling of "diagnostic[s] hotness", use the
new "diagnostics hotness" spelling in LLVM, which was added in
https://reviews.llvm.org/D34864.

Reviewers: anemet, davidxl

Reviewed By: anemet

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D34865

llvm-svn: 306862