Fix C++ thunk triggering for functions with default args which return a simple type

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.

Based on carbon-language#6151.
Follow up of carbon-language#6108.

Author: bricknerb

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 :