Fix case null on non-nullable type in unsafe nulls

[noti0na1]

Fix case null => ??? on non-nullable type in unsafe nulls, we should not get Match case Unreachable Warning.

Since we don't have import information after frontend, a StickyKey on match tree is used to pass Unsafe Nulls information to later PatternMatcher Phase.

def test1 =
  val s: String = ???
  s match
    case _: String =>
    // under unsafeNulls, we should not get Match case Unreachable Warning
    case null => 

[olhotak]

Defer this PR until after #14346 has been considered and either implemented or rejected.

[odersky]

I believe the sticky-key trick will not work if we compile from Tasty. At least we never used attachments to convey info from typer to a phase after pickling, to my knowledge. I believe we can do better by keeping the imports around until PatternMatcher, as suggested by #14346.

[olhotak]

LGTM