LibJS: Align codegen AwaitExpressions to YieldExpressions

We use generators in bytecode to approximate async functions, but the
code generated by AwaitExpressions did not have the value processing
paths that Yield requires, eg the `generator.throw()` path, which is
used by AsyncFunctionDriverWrapper to signal Promise rejections.

Userland/Libraries/LibJS/Bytecode/Interpreter.h

@@ -65,7 +65,11 @@ public:
         VERIFY(unwind_contexts().last().finalizer);
         jump(Label { *unwind_contexts().last().finalizer });
     }
-    void do_return(Value return_value) { m_return_value = return_value; }
+    void do_return(Value return_value)
+    {
+        m_return_value = return_value;
+        m_saved_exception = {};
+    }
 
     void enter_unwind_context(Optional<Label> handler_target, Optional<Label> finalizer_target);
     void leave_unwind_context();