[clang] [llvm] Remove 3dnow from X86TargetParser. (PR #99352)

[Simon Pilgrim via cfe-commits]

================
@@ -171,14 +171,14 @@ constexpr FeatureBitset FeaturesClearwaterforest =
 
 // Geode Processor.
 constexpr FeatureBitset FeaturesGeode =
-    FeatureX87 | FeatureCMPXCHG8B | FeatureMMX | Feature3DNOW | Feature3DNOWA;
----------------
RKSimon wrote:

I think we should add the define. If/when we eventually remove mm3dnow.h then the only way to access prfchwintrin.h is via x86intrin.h (which we already recommend) which can sometimes require the `__PRFCHW__` define:
https://github.com/llvm/llvm-project/blob/f6add66b720f85bf1092af7d6702b7397da57349/clang/lib/Headers/x86intrin.h#L17-L19

Also, we still have references to mm3dnow.h in prfchwintrin.h that probably need removing:
https://github.com/llvm/llvm-project/blob/f6add66b720f85bf1092af7d6702b7397da57349/clang/lib/Headers/prfchwintrin.h#L10-L12

https://github.com/llvm/llvm-project/pull/99352