The native HLSL compiler does not accept switch statements with an
empty statement list. The simplest way to accommodate this is to
simply remove them from the AST after parsing and some initial
pruning.

This is done by the new RemoveEmptySwitchStatements traverser. It
preserves init statements of switch statements in case they have side
effects. So for example

switch(++i) {}

gets translated to

++i;

BUG=angleproject:2206
TEST=angle_end2end_tests

Change-Id: I550a3c9b010a3566016bdfd93344ac30fd860604
Reviewed-on: https://chromium-review.googlesource.com/742922Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>