Vulkan: Refactor glslang wrapper's varying location assignment
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 <94770d7fe1cce6ea0791fa9927c1107de4d2e37b@google.com>
Reviewed-by: Tim Van Patten <c7814369a8d3d5671c484acc7b3215f1bf4221be@google.com>
Commit-Queue: Shahbaz Youssefi <582eeec4f08a2ccd61e3ef52115e6e7c571582de@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle