[clang] [llvm] [Headers][X86] Allow vector bitcast intrinsics to be used in constexpr (PR #167180)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@Red-RobinHood ping? You should be able to adopt the existing test coverage in avx-builtins.c for much of this

https://github.com/llvm/llvm-project/pull/167180