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

[Morris Hafner via cfe-commits]

mmha 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.

What I think should work and that's what I'm working on now is to enable CIR as a source language in the driver so we can compile and diagnose an invalid .cir file through clang instead of cir-opt. I'll follow up with another PR for that.

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