[clang] [CIR] Add the ability to detect if SwitchOp covers all the cases (PR #171246)

[Andy Kaylor via cfe-commits]

https://github.com/andykaylor commented:

This looks good. It could also be useful to know that the switch condition variable is an enum, for cases where it's not fully covered, but that could be a separate PR.

https://github.com/llvm/llvm-project/pull/171246