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

[Dave Bartolomeo via cfe-commits]

dbartol wrote:

The RFC has been approved to proceed by the Clang area team. I've updated the PR description to note a few open questions from the RFC, and how this PR chooses to answer each of those questions. 


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