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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman, could you take a look at this, please? If I can land this PR, I've got several less controversial SARIF changes stacked up behind it.

Whoops, this completely fell off my radar, thank you for the ping! I'll review shortly.

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