[clang] [X86] Emit Warnings for frontend options to enable knl/knm. (PR #75580)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

Sorry, we'd require both avx512f and avx512cd to work without avx512vl - but everything afterward (avx512bw, avx512dq, ....) we could assume avx512vl is enabled.

https://github.com/llvm/llvm-project/pull/75580