This change is in preparation for moving the linking logic to the
Implementation.

Introduces a ProgramLinkedResources class that is passed into the Impl
and holds linking-related info such as the UBOs, Varyings, etc.

BUG=angleproject:2208

Change-Id: I2ef0824b54bfb462c79d003bffe34e9cfad60d8a
Reviewed-on: https://chromium-review.googlesource.com/746204Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>