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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > > My preference: Either `DeprecatedStableIds` or `PreviousStableIds`.
> > 
> > 
> > `LegacyStableIds`? Whichever we do is going to need extensive documentation, so I don't have a strong preference. `Previous` strikes me the 'least', and `Old` is kinda bad. `Deprecated` has the advantage of actually being what is in SARIF, so I think I'm between `Legacy` and `Deprecated`, but I'm a touch of a coin-flip.
> 
> OK, I like `Legacy` better than either `Previous` or `Old`, so maybe a third voter can help us choose between `LegacyStableIds` and `DeprecatedStableIds`?

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.

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