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

[Oliver Hunt via cfe-commits]

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

Ok, so there's an existing `__ARM_FEATURE_PAUTH` but that does not cover the language features.
Naming-wise do we want something along the lines of `__ARM_FEATURE_PAUTH_LANG_SUPPORT__` ?

`__ARM_FEATURE_PAUTH` only indicates the target supports the features -- gcc doesn't provide anything more than that afaict

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