[perl #115768] improve (caller)[2] line numbers

[perl #115768] improve (caller)[2] line numbers

warn and die have special code (closest_cop) to find a nulled
nextstate op closest to the warn or die op, to get the line number
from it.  This commit extends that capability to caller, so that

  if (1) {
    foo();
  }
  sub foo { warn +(caller)[2] }

shows the right line number.