libstdc++: Clear RB tree after moving elements [PR103501]

libstdc++: Clear RB tree after moving elements [PR103501]

If the allocator-extended move constructor move-constructs each element
into the new container, the contents of the old container are left in
moved-from states. We cannot know if those states preserve the
container's ordering and uniqueness guarantees, so just erase all
moved-from elements.

libstdc++-v3/ChangeLog:

	PR libstdc++/103501
	* include/bits/stl_tree.h (_Rb_tree(_Rb_tree&&, false_type)):
	Clear container if elements have been moved-from.
	* testsuite/23_containers/multiset/allocator/103501.cc: New test.
	* testsuite/23_containers/set/allocator/103501.cc: New test.

(cherry picked from commit 056551414a328b427c4bf4955b9a3832f344685c)