nir/spirv: Add better location handling

nir/spirv: Add better location handling

Previously, our location handling was focussed on either no location
(usually implicit 0) or a builting.  Unfortunately, if you gave it a
location, it would blow it away and just not care.  This worked fine with
crucible and our meta shaders but didn't work with the CTS.  The new code
uses the "data.explicit_location" field to denote that it has a "final"
location (usually from a builtin) and, otherwise, the location is
considered to be relative to the base for that shader stage.