glsl: Rename ir_function_signature::builtin_info to builtin_avail.

glsl: Rename ir_function_signature::builtin_info to builtin_avail.

builtin_info was originally going to be a structure containing a bunch
of information, but after various rewrites, it turned into a boolean
availability predicate.

builtin_avail is a better name than builtin_info, since it doesn't
store any information other than availability.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>