[clang] [CIR] Add support for ternary operator as lvalue (PR #163580)

[Andy Kaylor via cfe-commits]

https://github.com/andykaylor approved this pull request.

Looks good to me. I have one nit about the `cir` namespace, but that can be put off to a broader change that fixes that problem everywhere.

https://github.com/llvm/llvm-project/pull/163580