[clang] [llvm] Remove support for 3DNow!, both intrinsics and builtins. (PR #96246)

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> I think we can just switch it to be a textual header; it no longer has any decls.

It does still have _MM3DNOW_H_INCLUDED, but we could probably remove that

https://github.com/llvm/llvm-project/pull/96246