Normative: Suppress GetMethod errors in IteratorClose

[zenparsing]

Fixes #1398

Current behavior:

If the "return" property of an iterator is not callable, a TypeError will be thrown from IteratorClose, regardless of the completion provided as an argument.

Proposed behavior:

If a "throw" completion is provided to IteratorClose, then it will rethrow that error even if the return property is not callable.

Test case:

let iter = {
  [Symbol.iterator]() { return this; },
  next() { return { value: 1, done: false }; },
  return: 0, // Not a function
};
try {
  for (let i of iter)
    throw 1; // Throw and trigger IteratorClose
} catch (e) {
  // Does this print "1" or "TypeError: 0 is not a function"?
  print(e);
}

Current engine behavior:

eshost -e '(() => { try { for (let i of { [Symbol.iterator]() { return this }, next() { return { value: 1, done: false }; }, return: 0 }) throw 1; } catch (e) { print(e) } })()'

#### ch
1

#### jsc
1

#### v8
TypeError: The iterator's 'return' method is not callable

#### sm
TypeError: property 'return' of iterator is not callable

#### xs
1

[littledan]

cc @LeszekSwirski @kmiller68

[shvaikalesh]

Tests PR: tc39/test262#2565

[bakkot]

LGTM other than the typographical issue.

[syg]

lgtm with convention nits fixed.