Treat aggregate constructors as having no side effects, which enables
pruning declarators that have a constructor initializer.

Some logic in RemoveUnreferencedVariables is fixed to make this work
correctly for structs. The bugs were previously not exposed since
constructors were treated as having side effects, but now that those
can be pruned the logic needs to be correct.

BUG=angleproject:2298
TEST=angle_unittests

Change-Id: I6fbe61a9e82065196baa29c200bf556fc21d8962
Reviewed-on: https://chromium-review.googlesource.com/856499Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>