[clang] [CIR] Implement builtin extractf (PR #170427)

[Henrich Lauko via cfe-commits]

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

Few last nits otherwise lgtm.

Also plese next time you can split unrelated changes like in this case changes to SelectOp and the builtin implementation.

https://github.com/llvm/llvm-project/pull/170427