Add comments and style fixes to <variant>

Add comments and style fixes to <variant>

	* include/std/variant: Adjust whitespace. Add comments.
	(_Multi_array): Leave primary template undefined.
	(_Multi_array<_Tp>): Define partial specialization for base case of
	recursion.
	(__gen_vtable_impl, __gen_vtable): Remove redundant && from type
	which is always a reference.
	(__gen_vtable::_S_apply()): Remove function, inline body into
	default member initializer.
	* testsuite/20_util/variant/visit.cc: Test with noncopyable types.

From-SVN: r270238