Validation and generation of transform feedback varyings that specify an
I/O block member are implemented in this change.  The GL backend is able
to pass the added tests.

Bug: angleproject:3606
Change-Id: I66d02bed8ca9161555d0d1e7a32ae9ef4d9e813f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2599952
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>