[clang] [clang][docs] Add documentation for EH codegen (PR #176236)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

I'm working on a design for EH and cleanup representation in CIR, and I generated this document to help me think through some things. It doesn't get into the very low-level details, but I think it could be useful as a guide to someone who is just learning the clang implementation.

I originally had it showing the IR output for the two examples, but that seemed a bit too verbose and not particularly relevant to what I intend to document, which is just the clang handling of the IR generation. The IR detail itself is sufficiently documented elsewhere.

https://github.com/llvm/llvm-project/pull/176236