[clang] [clang] Diagnose [[nodiscard]] return types in Objective-C++ (PR #142541)

[via cfe-commits]

================
@@ -272,6 +273,26 @@ QualType ObjCMessageExpr::getCallReturnType(ASTContext &Ctx) const {
   return Ctx.getReferenceQualifiedType(this);
 }
 
+std::pair<const NamedDecl *, const Attr *>
+ObjCMessageExpr::getUnusedResultAttr(ASTContext &Ctx) const {
+  // If the callee is marked nodiscard, return that attribute
+  if (const ObjCMethodDecl *MD = getMethodDecl())
+    if (const auto *A = MD->getAttr<WarnUnusedResultAttr>())
+      return {nullptr, A};
----------------
halbi2 wrote:

Yes, if this is changed to `return {MD, A}` then SemaObjC/method-warn-unused-attribute.m fails with
```
ignoring return value of type 'fee' declared with 'warn_unused_result' attribute
```
instead of the expected
```
ignoring return value of function declared with 'warn_unused_result' attribute
```

https://github.com/llvm/llvm-project/pull/142541