glsl: Parse the "binding" keyword and store it in ast_type_qualifier.

glsl: Parse the "binding" keyword and store it in ast_type_qualifier.

Nothing actually uses this yet.

v2: Remove >= 0 checks.  They'll be handled in later validation.

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