[clang] [clang] Make sure the same UsingType is searched and inserted (PR #79182)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

The change is correct. The problem is subtle though. It comes from the difference in behavior between the member and non-member Profile functions.

I think we could do better instead with a change which makes it harder to trip on this.

I think a simplification like this should work (untested):
```
diff --git a/clang/include/clang/AST/Type.h b/clang/include/clang/AST/Type.h
index ea425791fc97..3d411051084c 100644
--- a/clang/include/clang/AST/Type.h
+++ b/clang/include/clang/AST/Type.h
@@ -4729,13 +4729,12 @@ public:
   bool typeMatchesDecl() const { return !UsingBits.hasTypeDifferentFromDecl; }

   void Profile(llvm::FoldingSetNodeID &ID) {
-    Profile(ID, Found, typeMatchesDecl() ? QualType() : getUnderlyingType());
+    Profile(ID, Found, getUnderlyingType());
   }
   static void Profile(llvm::FoldingSetNodeID &ID, const UsingShadowDecl *Found,
                       QualType Underlying) {
     ID.AddPointer(Found);
-    if (!Underlying.isNull())
-      Underlying.Profile(ID);
+    Underlying.Profile(ID);
   }
   static bool classof(const Type *T) { return T->getTypeClass() == Using; }
 };
```

https://github.com/llvm/llvm-project/pull/79182