libstdc++-v3/ChangeLog:

	* include/bits/unique_ptr.h (operator<<): Define for C++20.
	* testsuite/20_util/unique_ptr/io/lwg2948.cc: New test.
	* testsuite/20_util/default_delete/48631_neg.cc: Adjust dg-error
	line number.
	* testsuite/20_util/default_delete/void_neg.cc: Likewise.

(cherry picked from commit 187da2ce31f13b2f75d5bb780e30ee364ead9d1d)