[backport] SI-7756 Uncripple refchecks in case bodies

In 65340ed4ad2e, parts of RefChecks were disabled when
we traversed into the results of the new pattern matcher.
Similar logic existed for the old pattern matcher, but in
that case the Match / CaseDef nodes still existed in the tree.

The new approach was too broad: important checks no longer
scrutinized the body of cases.

This commit turns the checks back on when it finds the remnants
of a case body, which appears as an application to a label def.

Conflicts:
	src/compiler/scala/tools/nsc/typechecker/RefChecks.scala

Cherry pick of 3df1d77fc984b976efa68098206e801cf3b83a9e
Merge pull request #3998 from retronym/backport/7756

[backport] SI-7756 Uncripple refchecks in case bodies