REPL/compiler leak `@unchecked` annotations in aliased assignments

[cswinter]

scala> {val x@y = 1; x} 
res0: Int @unchecked = 1

[allanrenucci]

Fixed in #3723. We already have a test for this in our test suite: https://github.com/lampepfl/dotty/blob/dd02cbc6b9114e6d5360e3f533d5b76f99eca7e5/compiler/test-resources/repl/patdef#L7