libstdc++: Explicitly default some copy ctors and assignments

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)