Clean up storing struct types in the symbol table 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>