Fix C++ thunk triggering for functions with default args which return a simple type (#6152)

Before this change, we wrongly ignore the decision to generate a thunk
for a function with default args by overriding this decision with the
fact the return type by itself doesn't require a thunk.
This causes not generating a thunk which leads to crashing in lowering.
Add tests that show that now thunk is generated in `check` and it no
longer crashes in `lower`.

Follow up of #6108.

toolchain/check/cpp/thunk.cpp

@@ -120,7 +120,8 @@ auto IsCppThunkRequired(Context& context, const SemIR::Function& function)
     if (return_type_id == SemIR::ErrorInst::TypeId) {
       return false;
     }
-    thunk_required = IsThunkRequiredForType(context, return_type_id);
+    thunk_required =
+        thunk_required || IsThunkRequiredForType(context, return_type_id);
   }
 
   for (auto param_id :