Avoid endless run-time recursion for copying single-element
	tuples where the element type is by-value constructible
	from any type.
 	* include/std/tuple (_NotSameTuple): New.
 	* include/std/tuple (tuple(_UElements&&...): Use it.
	* testsuite/20_util/tuple/cons/element_accepts_anything_byval.cc: New.

From-SVN: r236025