glsl: Construct gl_in with a location of -1.
glsl: Construct gl_in with a location of -1.

We use a location of -1 for variables which don't have their own
assigned locations--this includes ir_variables which represent named
interface blocks.  Technically the location assigned to gl_in doesn't
matter, since gl_in is only accessed via its members (which have their
own locations).  But it's nice to be consistent.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>
Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

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