attribs: Consider namespaces when comparing attributes

decl_attributes and comp_type_attributes both had code that
iterated over one list of attributes and looked for coresponding
attributes in another list.  This patch makes those lookups
namespace-aware.

gcc/
	* attribs.cc (find_same_attribute): New function.
	(decl_attributes, comp_type_attributes): Use it when looking
	up one list's attributes in another list.