Scala.js: Fix a codegen bug for switch matches in statement position.
Scala.js: Fix a codegen bug for switch matches in statement position.

The codegen chooses `jstpe.NoType` (`void`) over the declared type
of a `Match` node. That is normally fine, except if the declared
type was `Nothing`, in which case we must retain it, because the
enclosing statement might be demanding an expression of type
`Nothing`.

sjrd/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,lampepfl/dotty,dotty-staging/dotty,lampepfl/dotty