fix break inside of for loops that use next (#1436)

[ktegan]

This provides a simple solution to issue #1436 based on a the following comment "break" won't [work] until we do extra work to implement that using a flag and implementing "break" as "_for_break = true; continue;.

This would need code review because: there may be more elegant ways to do this, there are some details I'm not sure about (can print_cpp2() replace an emit() call?), and more testing for correctness would need to be done.

I think my biggest question is whether we should follow what the comment suggests or use an alternative approach where we define a default label for every loop and when there is a break that does not include a label add the label of the innermost loop to that break statement. If we used this labeled break solution the generated cpp1 code would use CPP2_CONTINUE_BREAK() to produce a goto statement instead of the solution in this PR which adds a variable and may require a comparison (check if _for_break is true) on every iteration through the loop.