[clang] [clang-tools-extra] [llvm] [X86] Emit Warnings for frontend options to enable knl/knm specific ISAs. (PR #75580)

[Simon Pilgrim via cfe-commits]

https://github.com/RKSimon approved this pull request.

LGTM - please make sure you adjust the commit message (the patch summary doesn't refer to removing just the knl/knm specific ISAs)

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