Merge TInterfaceBlock with TInterfaceBlockName, so that there are no
duplicate data structures for interface blocks. This is similar to the
refactoring that was already done to structs.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I67d2af6ccbe5344bddf9c99030d118fe532fbbd8
Reviewed-on: https://chromium-review.googlesource.com/805819Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>