libstdc++: Implement std::make_unique_for_overwrite

libstdc++: Implement std::make_unique_for_overwrite

This is the std::unique_ptr part of P1020R1 (as amended by P1973R1) for
C++20. The std::shared_ptr part still needs to be done.

libstdc++-v3/ChangeLog:

	* include/bits/unique_ptr.h (make_unique_for_overwrite): Define
	for C++20.
	* testsuite/20_util/unique_ptr/creation/array_neg.cc: Remove
	unused header. Adjust standard reference.
	* testsuite/20_util/unique_ptr/creation/for_overwrite.cc: New test.
	* testsuite/20_util/unique_ptr/creation/for_overwrite__neg.cc: New test.