Move count_attribute_slots() out of the linker and into glsl_type.

Move count_attribute_slots() out of the linker and into glsl_type.

Our previous justification for leaving this function out of glsl_type
was that it implemented counting rules that were specific to GLSL
1.50.  However, these counting rules also describe the number of
varying slots that Mesa will assign to a varying in the absence of
varying packing.  That's useful to be able to compute from outside of
the linker code (a future patch will use it from
ir_set_program_inouts.cpp).  So go ahead and move it to glsl_type.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>