glsl: set explicit_location correctly in lower_named_interface_blocks.

glsl: set explicit_location correctly in lower_named_interface_blocks.

When lower_named_interface_blocks lowers a built-in interface block
member to an ir_variable, it needs to set explicit_location in the
ir_variable.  Otherwise the linker gets confused and treats the
variable as a generic varying.

Fixes the following piglit tests, which were regressed by commit
63974c0 (glsl: Simplify the interface to
link_invalidate_variable_locations):
- clip-distance-bulk-copy
- clip-distance-in-bulk-read
- clip-distance-in-explicitly-sized
- clip-distance-in-param
- clip-distance-in-values
- core-inputs
- gs-redeclares-both-pervertex-blocks
- gs-redeclares-pervertex-in-only
- redeclare-pervertex-subset-vs-to-gs
- unsized-in-named-interface-block-gs
- unsized-in-named-interface-block-multiple
- unsized-in-unnamed-interface-block-gs
- unsized-in-unnamed-interface-block-multiple

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70820

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>