PR libstdc++/61947
	* include/std/tuple (_Head_base): Use allocator_arg_t parameters to
	disambiguate unary constructors.
	(_Tuple_impl): Pass allocator_arg_t arguments.
	* testsuite/20_util/tuple/61947.cc: New.
	* testsuite/20_util/uses_allocator/cons_neg.cc: Adjust dg-error line.

From-SVN: r213221