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

[Simon Pilgrim via cfe-commits]

RKSimon 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.

Removing the (incomplete) KNL/KNM specific features (ER/PF/etc) I don't think will be any major cause for concern, ideally we'd keep support at the assembly level only (which has been suggested for MMX as well at some point). But even that might not be necessary.

But what do you expect to happen if a KNL workstation attempts to use clang 19 with `-march=native`? Do we still enable it as a AVX512F+AVX512CD target or are you suggesting we degrade it to match x86-64-v3 arch spec?

How can the test cases then can be simplified? Are you suggesting that we merge more of the AVX512 attributes in X86.td (e.g. AVX512VL always enables BW/DQ  / VBMI2 enables VBMI etc?). 

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