This patch implements the C++2a proposal to allow incomplete types in
std::reference_wrapper, which was previously undefined.

The change cannot be implemented for earlier standards, because prior to
C++2a std::reference_wrapper has a weak result type, so must inspect the
template argument to see if it defines a nested result_type member. That
is deprecated (but still required) in C++17, and removed from C++2a.

The removal of the base class from reference_wrapper is a potential ABI
change, as it could alter the layout of a type which derives from
reference_wrapper<T> and from an empty type with _Weak_result_type<T> as
a base class.  Previously the repeated _Weak_result_type<T> base class
would have prevented the empty base-class optimization, but if
reference_wrapper<T> no longer derives from it, the empty class could be
placed at the same address as the reference_wrapper<T> base.  In
practice, the only types which derive from _Weak_result_type or from
_Reference_wrapper_base_memfun or any of its base classes are non-empty
types defined in libstdc++: std::reference_wrapper, std::function, and
std::_Bind. As they are non-empty types, they are not eligible for EBO
anyway.

	* include/bits/refwrap.h [__cplusplus > 201703L]
	(_Refwrap_base_arg1, _Refwrap_base_arg2, _Reference_wrapper_base)
	(_Reference_wrapper_base_memfun): Do not define for C++2a.
	(reference_wrapper): Do not derive from _Reference_wrapper_base_memfun
	for C++2a.
	(reference_wrapper::operator()): Add static assertion.
	* testsuite/20_util/reference_wrapper/incomplete.cc: New test.

From-SVN: r267866