Currently, when assigning varying locations, we assign in/out qualifiers
based on whether the varying name is present between different shader
stages.

This is broken for separate shader objects as there is possibly no pair
of shaders to be able to tell which is input and which is output (by
trying to find the same varying in a stage before or after the shader).

Bug: angleproject:3571
Change-Id: Id928a3eaa16e917c81bae0b3414c41842eb2b664
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1961993
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>