Merge WARN and CWARN cases in _bfd_generic_link_add_one_symbol

Merge WARN and CWARN cases in _bfd_generic_link_add_one_symbol

WARN has always been able to be handled by the CWARN case (WARN was
used when the previous symbols was undef, undefweak or common and thus
must be on the undefs list, so the CWARN test passes).  So let's merge
those two cases.

	* linker.c (WARN, CWARN): Collapse these states to WARN.
	(_bfd_generic_link_add_one_symbol): Use old CWARN case for
	new WARN.