[clang] [clang] [diagnostics] Stable IDs for Clang diagnostics (PR #168153)

[Dave Bartolomeo via cfe-commits]

dbartol wrote:

> I slightly prefer `Legacy` over `Deprecated`; deprecated tends to have a bit of a value judgement associated with it that I don't think exists here. But I don't have a strong preference, either is perfectly fine by me.

That's enough of a tie-breaker for me. Let's go with `Legacy`.

I'll merge my tblgen changes into this PR today, and then I'll merge once the implementation has been reviewed.



https://github.com/llvm/llvm-project/pull/168153