[clang] [llvm] [AArch64][clang][llvm] Allow +hcx to be specified as optional for Armv8.6 (PR #208019)

[Jonathan Thackray via cfe-commits]

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@@ -1061,7 +1061,7 @@ def HasV8_6aOps : Architecture64<8, 6, "a", "v8.6a",
   !listconcat(HasV8_5aOps.DefaultExts, [FeatureBF16, FeatureMatMulInt8])>;
 def HasV8_7aOps : Architecture64<8, 7, "a", "v8.7a",
   [HasV8_6aOps, FeatureXS, FeatureWFxT, FeatureHCX],
-  !listconcat(HasV8_6aOps.DefaultExts, [FeatureWFxT, FeatureSPE_EEF])>;
+  !listconcat(HasV8_6aOps.DefaultExts, [FeatureWFxT, FeatureHCX, FeatureSPE_EEF])>;
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jthackray wrote:

Yes, we do, because it has changed from to an `ExtensionWithMArch`, otherwise we get this compile error:
```
error: ExtensionWithMArch FeatureHCX is implied (mandatory) as a SubtargetFeature, but is not present in DefaultExts
```

https://github.com/llvm/llvm-project/pull/208019