Always-inlined annotated function does not get inlined in OClassic

[poechsel]

The following snippet behaves differently between OClassic and O3:

let[@inline always] loop_impl x =
  let rec loop x = if x then x else loop (not x) in
  loop x
;;

let wrap x = (loop_impl [@inlined always]) x

Works with -O3:

$ ocamlopt.opt -intf-suffix .no-mli -c -impl foo.ml -O3

Warns with -OClassic:

$ ocamlopt.opt -intf-suffix .no-mli -c -impl foo.ml -Oclassic
File "foo.ml", line 6, characters 13-44:
6 | let wrap x = (loop_impl [@inlined always]) x
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning 55 [inlining-impossible]: Cannot inline: 
  the optimizer did not know what function was being applied (is it marked [@inline never]?)
  (the full inlining stack was: foo.ml:6,13--44)\

This behavior is confusing: the function is marked as inline always, but does not actually get inlined by OClassic. There's two ways to go about that:

• Make OClassic inline this function, but this will make OClassic's behavior diverge from the historical closure behavior
• Do not inline this function, but emit a better warning saying something along the line of "Can't inline in OClassic because the function contained subfunctions"

[lthls]

For reference, Closure produces the following output:

File "foo.ml", line 6, characters 13-44:
6 | let wrap x = (loop_impl [@inlined always]) x
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning 55 [inlining-impossible]: Cannot inline: Function information unavailable

We should have enough information in classic mode to emit a better warning.