Implement resolutions of LWG 2399, 2400 and 2401.

Implement resolutions of LWG 2399, 2400 and 2401.

	* include/bits/shared_ptr.h (shared_ptr, weak_ptr): Define
	_Convertible alias template to simplify constraints.
	(shared_ptr(unique_ptr<TD>&&)): Constrain (LWG 2399).
	* include/bits/shared_ptr_base.h: Likewise.
	(_Sp_counted_deleter::_M_get_deleter()): Use addressof (LWG 2400).
	* include/std/functional (function::operator=(nullptr_t)): Add
	noexcept (LWG 2401).
	* 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: r217442