Decide when we should do exhaustivity checks

[DarkDimius]

scalac only checks sealed classes. We should consider if we indeed want to change this.
https://github.com/lampepfl/dotty/pull/1261/files#diff-3fbc86b62f6455f297c02b6711bc66b1R470

In case we indeed decide to change it, I propose to maintain scalac compatible behavior in -language:Scala2 mode.

[DarkDimius]

Spec says

If the selector of a pattern match is an instance of a sealed class, 
the compilation of pattern matching can emit warnings which 
diagnose that a given set of patterns is not exhaustive,
 i.e. that there is a possibility of a MatchError being raised at run-time.

This is the only reference to exhau* pattern matching spec.

[liufengyun]

dotty meeting: Follow the same approach as in scalac for now.

[liufengyun]

This is done in the #1364