[clang][PAC] Add ptrauth.h helpers for computing type discriminators (#163456)

Adds an additional helper that had not been upstreamed, and adds
documentation for both `ptrauth_type_discriminator` and
`ptrauth_function_pointer_type_discriminator`

Author: ojhunt

clang/docs/PointerAuthentication.rst

@@ -592,6 +592,36 @@ The result value is never zero and always within range for both the
 
 This can be used in constant expressions.
 
+``ptrauth_type_discriminator``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: c
+
+  ptrauth_type_discriminator(type)
+
+Compute the constant discriminator derived from the given type, as is computed
+for automatically type diversified schemas.
+
+``type`` must be a type. The result has the type ``ptrauth_extra_data_t``.
+
+This can be used in constant expressions.
+
+``ptrauth_function_pointer_type_discriminator``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: c
+
+  ptrauth_function_pointer_type_discriminator(function_type)
+
+Compute the constant discriminator derived from the provided function type, for
+use in contexts where the default function authentication schema. If function
+pointer type diversity is enabled, this is equivalent to
+`ptrauth_type_discriminator(function_type)`, if it is not enabled this is `0`.
+
+``function_type`` must be a function type. The result has the type ``ptrauth_extra_data_t``.
+
+This can be used in constant expressions.
+
 ``ptrauth_strip``
 ^^^^^^^^^^^^^^^^^
 

clang/lib/Headers/ptrauth.h

@@ -241,6 +241,18 @@ typedef __UINTPTR_TYPE__ ptrauth_generic_signature_t;
 #define ptrauth_type_discriminator(__type)                                     \
   __builtin_ptrauth_type_discriminator(__type)
 
+/* Compute the constant discriminator used by Clang to sign pointers with the
+   given C function pointer type.
+
+   A call to this function is an integer constant expression. */
+#if __has_feature(ptrauth_function_pointer_type_discrimination)
+#define ptrauth_function_pointer_type_discriminator(__type)                    \
+  __builtin_ptrauth_type_discriminator(__type)
+#else
+#define ptrauth_function_pointer_type_discriminator(__type)                    \
+  ((ptrauth_extra_data_t)0)
+#endif
+
 /* Compute a signature for the given pair of pointer-sized values.
    The order of the arguments is significant.
 
@@ -372,6 +384,8 @@ typedef __UINTPTR_TYPE__ ptrauth_generic_signature_t;
   })
 
 #define ptrauth_type_discriminator(__type) ((ptrauth_extra_data_t)0)
+#define ptrauth_function_pointer_type_discriminator(__type)                    \
+  ((ptrauth_extra_data_t)0)
 
 #define ptrauth_sign_generic_data(__value, __data)                             \
   ({                                                                           \