c++: Exported using decls

[Nathan Sidwell]

With modules we	need to	record whethe a	(namespace-scope) using	decl
is exporting the named entities.  Record this on the OVERLOAD marking
the used decl.

	gcc/cp/
	* cp-tree.h (OVL_EXPORT): New.
	(class ovl_iterator): Add get_using, exporting_p.
	* tree.c (ovl_insert): Extend using_or_hidden meaning to include
	an exported using.

pushed to trunk
-- 
Nathan Sidwell
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