glsl: Keep track of location for interface block fields.

This patch adds a "location" element to struct glsl_struct_field, so
that we can keep track of the gl_varying_slot associated with each
built-in geometry shader input.

In lower_named_interface_blocks, we use this value to populate the
"location" field in the ir_variable that stores each geometry shader
input.

Fixes piglit test spec/glsl-1.50/execution/geometry/core-inputs.