[clang] [llvm] [clang] Deduplicate builtin diagnostic descriptions (PR #202624)

[David Zbarsky via cfe-commits]

dzbarsky wrote:

> Apologies but this fell off my radar when I enabled squash and merge. Can you catch the changes up to ToT and I'll try to get them landed again?

OK, done. Let's get this merged? :)

https://github.com/llvm/llvm-project/pull/202624