glsl: add subroutine index qualifier support

glsl: add subroutine index qualifier support

ARB_explicit_uniform_location allows the index for subroutine functions
to be explicitly set in the shader.

This patch reduces the restriction on the index qualifier in
validate_layout_qualifiers() to allow it to be applied to subroutines
and adds the new subroutine qualifier validation to ast_function::hir().

ast_fully_specified_type::has_qualifiers() is updated to allow the
index qualifier on subroutine functions when explicit uniform locations
is available.

A new check is added to ast_type_qualifier::merge_qualifier() to stop
multiple function qualifiers from being defied, before this patch this
would cause a segfault.

Finally a new variable is added to ir_function_signature to store the
index. This value is validated and the non explicit values assigned in
link_assign_subroutine_types().

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>