libstdc++: Simplify std::aligned_storage and fix for versioned namespace [PR61458]

This simplifies the implementation of std::aligned_storage. For the
unstable ABI it also fixes the bug where its size is too large when the
default alignment is used. We can't fix that for the stable ABI though,
so just add a comment about the bug.

libstdc++-v3/ChangeLog:

	PR libstdc++/61458
	* doc/doxygen/user.cfg.in (GENERATE_BUGLIST): Set to NO.
	* include/std/type_traits (__aligned_storage_msa): Remove.
	(__aligned_storage_max_align_t): New struct.
	(__aligned_storage_default_alignment): New function.
	(aligned_storage): Use __aligned_storage_default_alignment for
	default alignment. Replace union with a struct containing an
	aligned buffer. Improve Doxygen comment.
	(aligned_storage_t): Use __aligned_storage_default_alignment for
	default alignment.