Fix Rust 1.66.1 opaque-hidden-inferred-bound error

Fix Rust 1.66.1 opaque-hidden-inferred-bound error

Summary:
Fixes:

  error: opaque type `impl IntoIterator<Item = hhbc::Entry<hhbc::TypedValue<'arena>, hhbc::TypedValue<'arena>>, IntoIter = impl ExactSizeIterator + 'arena> + 'arena` does not
  satisfy its associated type bounds
     --> hphp/hack/src/hackc/emitter/constant_folder.rs:550:50
      |
  550 | ) -> impl IntoIterator<Item = DictEntry<'arena>, IntoIter = impl ExactSizeIterator + 'arena> + 'arena
      |                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
      = note: `-D opaque-hidden-inferred-bound` implied by `-D warnings`

  error: aborting due to previous error

This fix is backwards compatible with the current rustc 1.65.0.

Reviewed By: bolinfest

Differential Revision: D43033753

fbshipit-source-id: f5008f69cc992a7a5a17881ba7073a6210a3f9a7