mesa: add fragdata_arrays list to gl_shader

mesa: add fragdata_arrays list to gl_shader

This is required to store information about fragdata arrays, currently
these variables get lost and cannot be retrieved later in sensible way
for program interface queries. List will be utilized by next patch.

Patch also modifies opt_dead_builtin_varyings pass to build list when
lowering fragdata arrays. This is identical approach as taken with
packed varyings pass.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Marta Lofstedt <marta.lofstedt@intel.com>