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

[Anatoly Trosinenko via cfe-commits]

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@@ -263,6 +294,32 @@ typedef __UINTPTR_TYPE__ ptrauth_generic_signature_t;
 #define ptrauth_sign_generic_data(__value, __data)                             \
   __builtin_ptrauth_sign_generic_data(__value, __data)
 
+/* Define some standard __ptrauth qualifiers used in the ABI. */
+#define __ptrauth_function_pointer(__typekey)                                  \
+  __ptrauth(ptrauth_key_function_pointer, 0, __typekey)
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atrosinenko wrote:

`__type_discriminator` was probably intended here, as the key is hardcoded
```suggestion
#define __ptrauth_function_pointer(__type_discriminator)                       \
  __ptrauth(ptrauth_key_function_pointer, 0, __type_discriminator)
```

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