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

[David Zbarsky via cfe-commits]

dzbarsky wrote:

> An existing connection was forcibly closed by the remote host.

@AaronBallman "An existing connection was forcibly closed by the remote host. " smells to me like an infra flake coupled with lack of client retry, I'm tempted to roll the dice here. I don't have permissions, can you please hit the merge button? :) 

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