Fix --no-as-needed when shared library is listed twice on the command line.

Fix --no-as-needed when shared library is listed twice on the command line.

When a shared library is listed twice on the command line, the linker
ignores the second mention. If the first mention is in the scope of
an --as-needed option, and the second one is under the scope of a
--no-as-needed option, the --no-as-needed should take effect, but
doesn't.  This patch keeps track of the objects we've already seen,
and updates the --as-needed flag so that if a shared object is ever
seen with --no-as-needed, it will be marked as such.

gold/
	PR gold/18859
	* object.cc (Input_objects::add_object): Store objects in a map,
	indexed by soname; update as-needed flag when necessary.
	* object.h (Object::clear_as_needed): New method.
	(Input_objects::so_names_): Change from set to map.