glsl: Add a new ast_type_qualifier::has_storage() method.

glsl: Add a new ast_type_qualifier::has_storage() method.

This makes it easy to check if any storage qualifiers are set.

"centroid" is not considered a storage qualifier.  In the old language
rules, you can't specify "centroid" by itself; it's always "centroid
in", "centroid out", or "centroid varying."  So one of the other storage
qualifiers will always be set; there's no need to specifically check for
centroid.

In the new 4.20 rules, centroid is an auxiliary storage qualifier, not a
storage qualifier.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>