Indexed fields in classes can now be initialized using iterators
and a spread syntax, e.g.:

  class Foo {
    length: Smi;
    elements[length]: Object;
  }

  new Foo{length: 5, elements: ...iter};

where iter implements Torque's iterator protocol. This protocol
requires the definition of a method with the following signature:

  Next(): <type> labels NoMore;

Where <type> is the Torque type of the values to be iterated.
In the case of indexed field initialization, the type must be
the field's type or a subtype thereof.

Field initialization with spread is desugared into a loop that
calls the spread iterator's Next method and assigns each
returned value in order to the corresponding indexed field
element.

The general machinery for the spread syntax has been added to
the ast and parser, however, it can currently only be used in
the specific context of indexed field initialization. Spread
operators used in any other context will cause an error.

Bug: v8:7793
Change-Id: If071e61db8166573c28d13318879c88ba96f6d98
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1550407
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60639}