Varying location is determined based on XFB location. If the transform
feedback stage is Vertex, there is no problem. But if the transform
feedback stage is GS or TES, previous implementation can cause shader
interface mismatch.

Bug: angleproject:5557
Test: KHR-GLES32.core.tessellation_shader.tessellation_control_to_tessellation_evaluation.gl_in
Change-Id: I1ecc7913a178c9e674307c528d1bdf13aabcb665
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2784713Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>