Test line number for (??{$str}) regexp warnings

Test line number for (??{$str}) regexp warnings

If the (??{}) does not occur on the same line as the match operator
(e.g., if we have /foo$qr/ and $qr has (??{}) in it), we need to
make sure warnings that occur when the return value of (??{}) is
compiled use the same line number as the /.../, since conceptually
it is part of matching, not part of returning.

I tried breaking this and making the line number wrong, but all
tests passed, showing that it is untested.  Most regcomp tests are in
t/re/reg_mesg.t, but that file does not have infrastructure for check-
ing line numbers.