Fix #4922: unapplySeq mustn't allow arrays

[Blaisorblade]

Fix #4922 with the "obvious" fix.

If this triggers cyclic errors, the extra check could be moved to PostTyper.

The error message could be improved, since it doesn't mention the specific reason why SQ isn't a valid result type.

12 |      case A(xs: _*) => xs
   |           ^^^^^^^^^
   | SQ is not a valid result type of an unapplySeq method of an extractor()
12 |      case A(xs: _*) => xs
   |                        ^^
   |                        not found: xs

Beware this was a 5-minute job, so haven't thought hard about it.

[liufengyun]

LGTM

[Blaisorblade]

Merging as it's an improvement, but @lrytz any feedback is still welcome!