Fix #9266: Offer rewrites for parentheses in implicit closures

[odersky]

I don't have the time to write a proper test for this. I verified manually
that it does what it should:

-- Migration Warning: i9266.scala:3:23 -----------------------------------------
3 |def test = { implicit x: Int => x + x }
  |                       ^
  |This syntax is no longer supported; parameter needs to be enclosed in (...)
  |This construct can be rewritten automatically under -rewrite.
1 warning found

[anatoliykmetyuk]

I've modified the test to test the warning message.

[odersky]

The thing se could test is that -rewrite works as expected. But it's not a big deal either way.