libstdc++: Restore __gnu_debug::array [PR100682]

libstdc++: Restore __gnu_debug::array [PR100682]

As the PR points out, we removed the debug version of std::array without
any period of deprecation. Although std::array contains all the actual
debug checks now, removing the <debug/arrray> header breaks any code
that was using that explicitly. The manual still lists doing that as
supported.

This restores the <debug/array> header, but simply defines
__gnu_debug::array as an alias for std::array, and declares the alias
with the deprecated attribute. The docs are updated to match.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	PR libstdc++/100682
	* doc/xml/manual/debug_mode.xml: Update documentation about
	debug capability of std::array.
	* doc/html/*: Regenerate.
	* include/debug/array: New file.