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

[Jonathan Wakely]

This fixes an invalid assumption that std::atomic<T> can default
construct a T. It also fixes some variables called "tmp" with
non-uglified names.

	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.

Tested powerpc64le-linux, committed to trunk.

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