Instead of storing struct symbols as TVariable objects, store them
as TStructure objects. This way struct type symbols don't need to
store bogus data that's associated with arbitrary types, and on the
other hand the name and unique id of struct types are only stored in
a single place. This change is a refactoring with no effect on
generated output.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I173793e0cc77a890cdac6868d72d9f275ac9461a
Reviewed-on: https://chromium-review.googlesource.com/793814
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>