[PAC][compiler-rt][UBSan] Strip signed vptr instead of authenticating it (llvm#100153)

vptr cannot be authenticated without knowing the class type if it was
signed with type discrimination.

Co-authored-by: Oliver Hunt <[email protected]>

Author: ahatanak

compiler-rt/lib/ubsan/ubsan_type_hash_itanium.cpp

@@ -207,7 +207,7 @@ struct VtablePrefix {
   std::type_info *TypeInfo;
 };
 VtablePrefix *getVtablePrefix(void *Vtable) {
-  Vtable = ptrauth_auth_data(Vtable, ptrauth_key_cxx_vtable_pointer, 0);
+  Vtable = ptrauth_strip(Vtable, ptrauth_key_cxx_vtable_pointer);
   VtablePrefix *Vptr = reinterpret_cast<VtablePrefix*>(Vtable);
   VtablePrefix *Prefix = Vptr - 1;
   if (!IsAccessibleMemoryRange((uptr)Prefix, sizeof(VtablePrefix)))