[clang] [CIR] Upstream CIR codegen for undef x86 builtins (PR #167945)

[Thibault Monnier via cfe-commits]

Thibault-Monnier wrote:

I am in the process of adding the tests I forgot to upstream. This is also the case for the mxcsr builtins PR.

https://github.com/llvm/llvm-project/pull/167945