[ORE] Unify spelling as "diagnostics hotness"

Summary:
To enable profile hotness information in diagnostics output, Clang takes
the option `-fdiagnostics-show-hotness` -- that's "diagnostics", with an
"s" at the end. Clang also defines `CodeGenOptions::DiagnosticsWithHotness`.

LLVM, on the other hand, defines
`LLVMContext::getDiagnosticHotnessRequested` -- that's "diagnostic", not
"diagnostics". It's a small difference, but it's confusing, typo-inducing, and
frustrating.

Add a new method with the spelling "diagnostics", and "deprecate" the
old spelling.

Reviewers: anemet, davidxl

Reviewed By: anemet

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D34864

llvm-svn: 306848