[clang] [clang][PAC][NFC] Provide addition support macros to ptrauth.h (PR #161027)

[Oliver Hunt via cfe-commits]

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@@ -199,6 +199,25 @@ typedef __UINTPTR_TYPE__ ptrauth_generic_signature_t;
   ptrauth_auth_and_resign(__value, __old_key, __old_data,                      \
                           ptrauth_key_function_pointer, 0)
 
+/* Cast a value to the given type without changing any signature.
+
+   The type must be a pointer sized type compatible with the __ptrauth
+   qualifier.
+   The value must be an expression with a non-address diversified pointer
+   authentication schema, and will be converted to an rvalue prior to the cast.
+   The result has type given by the first argument.
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ojhunt wrote:

alas no, and these have to work in C

https://github.com/llvm/llvm-project/pull/161027