[ORE] Remove old "diagnostic hotness" spelling

Summary:
Depends on https://reviews.llvm.org/D34865.

With the Clang uses of the old spelling having been removed in
https://reviews.llvm.org/D34865, get rid of the old "diagnostic hotness"
spellings in favor of the new "diagnostics hotness".

Reviewers: anemet, davidxl

Reviewed By: anemet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34866

llvm-svn: 306866