[clang] [llvm] Remove mmx 3dnow (PR #96246)

[James Y Knight via llvm-commits]

================
@@ -7,151 +7,16 @@
  *===-----------------------------------------------------------------------===
  */
 
+// 3dNow intrinsics are no longer supported, and this header remains only as a
+// stub for users who were including it to get to _m_prefetch or
+// _m_prefetchw. Such uses should prefer x86intrin.h.
----------------
jyknight wrote:

Looking at this again, I do think it's reasonable to emit a `#warning` here.

I was initially worried that might cause issues for users, but there's so very very few users of anything 3dnow-related to start with, and the ones I found generally conditioned the include on `__3dNOW__` being defined, anyhow.

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