attribs: Namespace-aware lookup_attribute_spec

attribute_ignored_p already used a namespace-aware query
to find the attribute_spec for an existing attribute:

      const attribute_spec *as = lookup_attribute_spec (TREE_PURPOSE (attr));

This patch does the same for other callers in the file.

gcc/
	* attribs.cc (comp_type_attributes): Pass the full TREE_PURPOSE
	to lookup_attribute_spec, rather than just the name.
	(remove_attributes_matching): Likewise.