Constrain std::shared_ptr assignment and resetting

Constrain std::shared_ptr assignment and resetting

	* include/bits/shared_ptr.h (_Assignable): New alias template.
	(shared_ptr::operator=(const shared_ptr<_Tp1>&))
	(shared_ptr::operator=(shared_ptr<_Tp1>&&))
	(shared_ptr::operator=(unique_ptr<_Tp1>&&)): Constrain with
	_Assignable.
	* include/bits/shared_ptr_base.h (_Assignable): New alias template.
	(__shared_ptr::operator=(const __shared_ptr<_Tp1>&))
	(__shared_ptr::operator=(__shared_ptr<_Tp1>&&))
	(__shared_ptr::operator=(unique_ptr<_Tp1>&&)): Constrain with
	_Assignable.
	(__shared_ptr::reset(_Tp1*), __shared_ptr::reset(_Tp1*, _Deleter))
	(__shared_ptr::reset(_Tp1*, _Deleter, _Alloc)): Constrain with
	_Convertible.
	* testsuite/20_util/shared_ptr/cons/43820_neg.cc: Change dg-error to
	match on any line.
	* testsuite/20_util/shared_ptr/cons/void_neg.cc: Likewise.
	* testsuite/20_util/shared_ptr/assign/sfinae.cc: New test.
	* testsuite/20_util/shared_ptr/assign/shared_ptr_neg.cc: Update
	expected errors. Remove unnecessary code.
	* testsuite/20_util/shared_ptr/modifiers/reset_sfinae.cc: New test.

From-SVN: r239898