[clang] Add ABI-defined key assignments to ptrauth.h. (llvm#93901)

This is the current key assignment defined by the arm64e architecture
on Darwin platforms, and also matches the intended key assignments
in the initial ELF adopters.  This can be made more generic if needed,
by e.g., defining these in the preprocessor instead.

Author: ahmedbougacha

clang/lib/Headers/ptrauth.h

@@ -15,6 +15,19 @@ typedef enum {
   ptrauth_key_asib = 1,
   ptrauth_key_asda = 2,
   ptrauth_key_asdb = 3,
+
+  /* A process-independent key which can be used to sign code pointers. */
+  ptrauth_key_process_independent_code = ptrauth_key_asia,
+
+  /* A process-specific key which can be used to sign code pointers. */
+  ptrauth_key_process_dependent_code = ptrauth_key_asib,
+
+  /* A process-independent key which can be used to sign data pointers. */
+  ptrauth_key_process_independent_data = ptrauth_key_asda,
+
+  /* A process-specific key which can be used to sign data pointers. */
+  ptrauth_key_process_dependent_data = ptrauth_key_asdb,
+
 } ptrauth_key;
 
 /* An integer type of the appropriate size for a discriminator argument. */