glsl: store uniform slot id in var location field

glsl: store uniform slot id in var location field

This will allow us to access the uniform later on without resorting to
building a name string and looking it up in UniformHash.

V3: remove line wrap change from this patch

V2: store slot number for all non-UBO uniforms to make code more
consitent, renamed explicit_binding to explicit_location and added
comment about what it does. Store the location at every shader stage.
Updated data.location comments in ir/nir.h.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>