libstdc++: Fix some -Wmismatched-tags warnings

libstdc++: Fix some -Wmismatched-tags warnings

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr_atomic.h (atomic): Change class-head
	to struct.
	* include/bits/stl_tree.h (_Rb_tree_merge_helper): Change
	class-head to struct in friend declaration.
	* include/std/chrono (tzdb_list::_Node): Likewise.
	* include/std/future (_Task_state_base, _Task_state): Likewise.
	* include/std/scoped_allocator (__inner_type_impl): Likewise.
	* include/std/valarray (_BinClos, _SClos, _GClos, _IClos)
	(_ValFunClos, _RefFunClos): Change class-head to struct.