glsl/linker: outputs in the same location must share auxiliary storage

glsl/linker: outputs in the same location must share auxiliary storage

From ARB_enhanced_layouts:

"[...]when location aliasing, the aliases sharing the location
  must have the same underlying numerical type (floating-point or
  integer) and the same auxiliary storage and
  interpolation qualification.[...]"

Add code to the linker to validate that aliased locations do
have the same aux storage.

Fixes:
KHR-GL45.enhanced_layouts.varying_location_aliasing_with_mixed_auxiliary_storage

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>