libstdc++: _Rb_tree code cleanup, remove lambdas

Use new template parameters to replace usage of lambdas to move or not
tree values on copy.

libstdc++-v3/ChangeLog:

	* include/bits/move.h (_GLIBCXX_FWDREF): New.
	* include/bits/stl_tree.h: Adapt to use latter.
	(_Rb_tree<>::_M_clone_node): Add _MoveValue template parameter.
	(_Rb_tree<>::_M_mbegin): New.
	(_Rb_tree<>::_M_begin): Use latter.
	(_Rb_tree<>::_M_copy): Add _MoveValues template parameter.
	* testsuite/23_containers/map/allocator/move_cons.cc: New test.
	* testsuite/23_containers/multimap/allocator/move_cons.cc: New test.
	* testsuite/23_containers/multiset/allocator/move_cons.cc: New test.
	* testsuite/23_containers/set/allocator/move_cons.cc: New test.