[Test] Fix expected return value in typed_throws_thunks.swift

For empty errors, we expect `undef` to eb returned

Author: drexin

test/IRGen/typed_throws_thunks.swift

@@ -99,7 +99,7 @@ protocol P2 {
 // CHECK-SAME:  )
 // CHECK-SAME:  {
 // CHECK:       failure:
-// CHECK-NEXT:    ret void
+// CHECK-NEXT:    ret i8 undef
 // CHECK:       success:
 // CHECK-NEXT:    ret i8 %{{.*}}
     func h() throws(E) -> Int8