glsl: Use merge_qualifier() when processing qualifier lists.

glsl: Use merge_qualifier() when processing qualifier lists.

Most of ast_type_qualifier is simply a bitfield (represented as a
structure of unsigned:1 bits in a union with an unsigned).  However, it
also contains ARB_explicit_attrib_location's location/index fields.

In the past, this has worked by simply returning the layout qualifier's
ast_type_qualifier and merging the other bits into it.  However, that's
not obvious until you break it by switching $1 and $2.

Using merge_qualifier() copies them appropriately, and also properly
overrides layout qualifiers.  It also checks for duplicate qualifiers,
which renders some of the checks in the previous patch unnecessary.
However, those checks provide better error messages, such as "Duplicate
interpolation qualifier", rather than just "duplicate qualifier".

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