Regression in staging level checks

[nicolasstucki]

Since #4363, the following two cases break

39 |    '(Option(4) match { case Some(a) => a; case None => 1 }),
   |                                        ^
   |                                        access to value a from wrong staging level:
   |                                         - the definition is at level 0,
   |                                         - but the access is at level 1.

39 |    '{ case class Foo() },
   |                       ^
   |                       access to package _root_ from wrong staging level:
   |                        - the definition is at level 0,
   |                        - but the access is at level 1.