Vulkan: Refactor glslang wrapper's varying location assignment Instead of multiple ifs that test for various combination of shader stages and calling AssignVaryingLocations for the appropriate pairs, that function loops over all stages and assigns a location to the varying in any stage it is active. This change will help keep this code simpler with the introduction of tessellation shaders. Bug: angleproject:3571 Change-Id: Iaf3ee67990bd219080314237b314bd2770a43085 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1961096 Reviewed-by: Cody Northrop <cnorthrop@google.com> Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>