Add logic to check the program or the pipeline before erroring out when
validating glBeginTransformFeedeback.

Bug: angleproject:5557
Test: ProgramPipelineXFBTest31.VaryingIOBlockSeparableProgramWithXFB*
Change-Id: I0df8a8d87b3632745bc91dc2673f2fac31c6cdb1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2743440Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>