[clang] [CIR] Add MLIR diagnostic handler ahead of verification (PR #165838)

[Bruno Cardoso Lopes via cfe-commits]

bcardosolopes wrote:

> The reason I didn't add an explicit test was that there's no way to explicitly trigger an MLIR error from C/C++ source code with clang. The same is true for LLVM errors and OGCG as far as I know.

How about `errorNYI` output? 

https://github.com/llvm/llvm-project/pull/165838