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

[Balázs Benics via cfe-commits]

https://github.com/steakhal commented:

Hey, I made it! I left a couple of tiny comments inline, but overall the PR looks very solid and well thought through! I was pleased to review this.

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