We put pruning literal statements and pruning empty declarations in
the same traverser, as some of the required logic is the same. This
pruning of no-ops is always done as one of the first processing steps
after parsing, so further processing of the AST is simpler.

Since we now prune pure literals before removing no-op cases from the
end of switch statements, we also don't need any sort of special
handling for switch statements in pruning pure literals.

BUG=angleproject:2181
TEST=angle_unittests

Change-Id: I2d86efaeb80baab63ac3cc803f3fd9e7ec02908a
Reviewed-on: https://chromium-review.googlesource.com/727803Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>