libstdc++: Fix tests broken by C++23 P2266R3 "Simpler implicit move"

libstdc++: Fix tests broken by C++23 P2266R3 "Simpler implicit move"

In C++23 mode these tests started to FAIL because an rvalue reference
parameter can no longer be bound to an lvalue reference return type. As
confirmed by Ville (who added these tests) the problem overloads are not
intended to be called, and only exist to verify that they don't
interfere with the intended behaviour. This changes the function bodies
to just throw, so that the tests will fail if the function is called.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_ostream/inserters_other/char/6.cc:
	Change body of unused operator<< overload to throw if called.
	* testsuite/27_io/basic_ostream/inserters_other/wchar_t/6.cc:
	Likewise.