[llvm] Change type of DiagnosticHandlerTy (PR #86504)

[Amy Kwan via llvm-commits]

amy-kwan wrote:

> @AaronBallman @amy-kwan Can we merge it? I'll watch the issue for a couple of days for any fallout breakages.

Yes, I am OK if we merge this. LGTM.

https://github.com/llvm/llvm-project/pull/86504