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

[Freddy Ye via cfe-commits]

FreddyLeaf wrote:

Let me first paste reasons I listed before:
1. INTEL has officially announced these products’ EOL on about Aug. 2017
1. Even for now, clang/llvm’s supports on these products are incomplete. For example, knm targets has AVX5124FMAPS instructions, while its intrinsic and assembly support is missing. And it is weird that avx5124fmaps is still listed at llvm/include/llvm/TargetParser/X86TargetParser.def.
1. It sometimes leads to bad performance by wrong usage of options.
1. We can claim this change on release notes to notify users intentionally support these targets to use older releases.


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