The debug mode tests for map/set's iterators construct empty
containers, making the code after the first increment meaningless.
It's never executed since the tests exit earlier.
It doesn't seem to be intentional, so the patch makes the tests
to construct containers that include at least one element.
Reviewed By: curdeius, Quuxplusone
Differential Revision: https://reviews.llvm.org/D100029
4a292eda25