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

[Freddy Ye via cfe-commits]

FreddyLeaf wrote:

@RKSimon  @phoebewang  Thanks comments! knm has AVX512_VPOPCNTDQ, I guess we also require it work without avx512vl? And we are going to keep -march/mtune/mcpu support for knl/knm, but removing some specific ISA's intrinsic and lowering supports? Did I get your point wrong?

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