* include/bits/shared_ptr.h (weak_ptr): Add move constructor and
	assignment.
	* include/bits/shared_ptr_base.h (__weak_count, __weak_ptr): Likewise.
	Use nullptr and injected class name.
	* testsuite/20_util/shared_ptr/cons/43820_neg.cc: Adjust dg-error.
	* testsuite/20_util/shared_ptr/cons/void_neg.cc: Adjust dg-error.

From-SVN: r217413