PATCH: [perl #121816] Add warning for repetition x < 0

I consider this experimental, so that if code breaks as a result, we
will remove it.

I chose the numeric warnings category.  But misc or a new subcategory of
numeric might be better choices.

There is also the issue if someone is calculating the repeat count in
floating point and gets something that would be 0 if there were infinite
precision, but ends up being a very small negative number.  The current
implementation will warn on that, but probably shouldn't.  I suspect that
this would be extremely rare in practice.