Add support for labeled while loops

Fixes #12643

> Say!
> I like labelled breaks/continues!

I will use them with a `for` loop.
And I will use with a `loop` loop.
Say! I will use them ANYWHERE!
… _even_ in a `while` loop.

Because they're now supported there.
I'm not sure why `loop` loops ignore the body here while `for` loops don't - so I took what seemed to be the more conservative approach and treat `while` loops like `loop` loops (i.e. ignore the body and expression)