Ql/incomplete tostring

[yoff]

TODO:

• Some classes are restircted in their charpred to some branch types.
  We should take that into account when computing coverage.
• Some classes rely on onther classes for cover. Example is
  ReturnCompletion:

/**
 * A completion that represents evaluation of a statement or an
 * expression resulting in a return from a callable.
 */
class ReturnCompletion extends Completion {
  ReturnCompletion() {
    this = TReturnCompletion() or
    this = TNestedCompletion(_, TReturnCompletion(), _)
  }
  override ReturnSuccessor getAMatchingSuccessorType() { any() }
  override string toString() {
    // `NestedCompletion` defines `toString()` for the other case
    this = TReturnCompletion() and result = "return"
  }
}