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

[Freddy Ye via cfe-commits]

FreddyLeaf wrote:

> Can you give me a better idea of the stages you intend to follow with this. The patch title suggests removing all KNL/KNM handling but the patch itself looks to be just about the KNL/KNM specific features.

I have created a draft PR to do removal in next release: https://github.com/llvm/llvm-project/pull/76383. Comments are welcome there. For this PR, I added warnings to features only but not cpuname, mainly to reduce efforts. Especially when using `-march=native` on a knl machine, these warnings can also be emitted.

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