PR69301 don't assume atomic<T> can default construct T

PR69301 don't assume atomic<T> can default construct T

	PR libstdc++/69301
	* include/std/atomic (atomic<T>::load, atomic<T>::exchange): Use
	aligned buffer instead of default-initialized variable.
	* testsuite/29_atomics/atomic/69301.cc: New test.
	* include/experimental/memory (observer_ptr::release): Use reserved
	name.
	* include/ext/pointer.h (_Pointer_adapter::operator++(int))
	(_Pointer_adapter::operator--(int)): Likewise.

From-SVN: r244588