[clang] [PAC][clang] Implement address/type discrimination for VTT vtable pointers (PR #205808)

[Oliver Hunt via cfe-commits]

================
@@ -2221,13 +2222,19 @@ llvm::Value *ItaniumCXXABI::getVTableAddressPointInStructorWithVTT(
       CGF.Builder.CreateAlignedLoad(CGF.GlobalsVoidPtrTy, VTT,
                                     CGF.getPointerAlign());
 
-  if (auto &Schema = CGF.CGM.getCodeGenOpts().PointerAuth.CXXVTTVTablePointers) {
-    CGPointerAuthInfo PointerAuth = CGF.EmitPointerAuthInfo(Schema, VTT,
-                                                            GlobalDecl(),
-                                                            QualType());
+  if (auto &Schema = CGM.getCodeGenOpts().PointerAuth.CXXVTTVTablePointers) {
+    llvm::ConstantInt *TypeDesc = nullptr;
+    if (Schema.getOtherDiscrimination() ==
+        PointerAuthSchema::Discrimination::Type)
+      TypeDesc = llvm::ConstantInt::get(
+          CGM.IntPtrTy,
+          CGM.getContext().getPointerAuthVTablePointerDiscriminator(
----------------
ojhunt wrote:

Sorry, I wasn't clear here, and you're correct that per vtable encoding would indeed break things.

What I was actually trying to say was to semi-generalize the existing `getPointerAuthVTablePointerDiscriminator` to provide discriminators for the non-function primary base specific entries, e.g
```cpp
enum class VTableEnumWithGoodName { VTable, VTT, ... };
std::optional<PointerAuthQualifier> computeVTPointerAuthentication(const CXXRecordDecl *ThisClass, VTableEnumWithGoodName SlotKind) {
...
Discriminator = Context.getPointerAuthVTablePointerDiscriminator(PrimaryBase, SlotKind);

...
uint16_t getPointerAuthVTablePointerDiscriminator(const CXXRecordDecl *RD, SlotKind) {
...
  switch (SlotKind) {
  case vtable: mangleCXXVTable(RD);
  case vtt: mangleCXXVTT(RD);
...
}
```

Or something to that effect.

https://github.com/llvm/llvm-project/pull/205808