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.
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