glsl: Refactor uniform block parser rules.

glsl: Refactor uniform block parser rules.

The existing code has a lot of duplication; the only difference between
the two cases is whether we merge in an additional layout qualifier.

Apparently creating a layout_qualifieropt rule that can be empty causes
a lot of conflicts and confusion.  However, refactoring out the guts of
the ast_uniform_block creation works fine.

Reviewed-by: Carl Worth <cworth@cworth.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>