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

[Aaron Ballman via cfe-commits]

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

I'm happy with `__PTRAUTH__`; do we want it to expand to a particular value so we can change the value to have the macro mean a different feature set is enabled?

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