Support @ _* syntax under -language:Scala2

[DarkDimius]

Dotty requires x : _* syntax for repeated types, though scala required xs @ _*.
It would be good to add support for old syntax under compatibility flag and issue a warning.
Also see #1048

[liufengyun]

@DarkDimius I'd like to work on this issue.

To be clear, scala supports both xs @ _* and _*, so we need to support both in dotty as well, or just xs @ _*?