impl ExactSize for vec::MoveItems
auto merge of #17266 : Gankro/rust/vec-move, r=alexcrichton

Seems to correctly report exact size, so it should claim to do so formally.
syntax: use an index in CodeMap instead of Gc for ExpnInfo.
rustc: fix fallout from removing the use of Gc for ExpnInfo.
Fix fallout in tests from removing the use of Gc in ExpnInfo.