glsl: Handle most qualifier ordering in C code rather than the grammar.

glsl: Handle most qualifier ordering in C code rather than the grammar.

The GL_ARB_shading_language_420pack extension/GLSL 4.20 allow qualifiers
to be specified in (basically) any order.  In order to support this, we
can't hardcode the ordering restrictions in the grammar.

This patch alters the grammar to accept invariant, storage, layout, and
interpolation qualifiers in any order, but adds C code to enforce the
ordering requirements.  In the 420pack case, we should be able to simply
skip the error checks.

As a bonus, this also lets us generate decent error messages, rather
than Bison's awful "unexpected TOKEN" errors.

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