[clang] [clang][PAC] ptrauth_qualifier and ptrauth_intrinsic should only be available on Darwin (PR #153912)

[Oliver Hunt via cfe-commits]

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@@ -1528,6 +1528,9 @@ static void InitializePredefinedMacros(const TargetInfo &TI,
 #undef TARGET_OS
   }
 
+  if (LangOpts.PointerAuthIntrinsics)
+    Builder.defineMacro("__PTRAUTH__");
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ojhunt wrote:

I think @AaronBallman had an opinion on the `__` suffix - I am absolutely indifferent here and as demonstrated am terrible at names :D

https://github.com/llvm/llvm-project/pull/153912