Allows shaders that use GS/TS to have input varying variables.

Bug: angleproject:5409
Change-Id: Idfbb725a4eef9e55fdde51d804a891b05dd1daee
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2572879Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>