libstdc++: Explicitly default some copy ctors and assignments
The standard says that the implicit copy assignment operator is
deprecated for classes that have a user-provided copy constructor, and
vice versa.
libstdc++-v3/ChangeLog:
* include/bits/new_allocator.h (__new_allocator): Define copy
assignment operator as defaulted.
* include/std/complex (complex<float>, complex<double>)
(complex<long double>): Define copy constructor as defaulted.
(cherry picked from commit
008e439f34d4b356825a6c9b70245143f00bd353)