This unifies the variable handling for IO blocks. Previously the
gl_in block we treated as an "in block" similar to an interface block.

Bug: angleproject:5423
Change-Id: Idf2db4c209055fa9ffca567dab39c89464eba156
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2571966Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>