go/types, types2: move Checker.indexedElts into literals.go where it belongs
The function is only used by Checker.compositeLit.
Also, now its go/types source can be gerated from the types2 source.
No other code changes.
Change-Id: I88b7ad371d809a5d9bf8e635d9e003ba0a71ab78
Reviewed-on: https://go-review.googlesource.com/c/go/+/615635
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Tim King <taking@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>