CaseClause never made sense as an Expression; this CL allows us to
remove several UNREACHABLEs and slim down the representation of
CaseClause by removing its source position (which was only used
in prettyprinting).

The only real fallout of this change is that SourceRangeMap now
stores its keys as ZoneObject*, rather than AstNode*, but since
there's already compile time typechecking for inserting items
into the map this shouldn't cause any ill effects.

While modifying CaseClause, also removed the dead body_target()
accessor (and related member variable). Thus this CL overall
reduces the memory needed for each CaseClause by two words.

Bug: v8:6092
Change-Id: I0021c0590a69e29305c41ec6105c8824ae0cc25b
Reviewed-on: https://chromium-review.googlesource.com/639316Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47722}