Erase global_ by moving the modality to a [@globalized] attribute

[dvulakh]

Erasing modalities can cause ppxs that inspect them to fail. Consider:

open! Core

module A = struct
  type t = int
end

module B = struct
  type t =
    { global_ a : A.t
    ; b : string
    }
  [@@deriving globalize]
end

As written, this code compiles. If global_ is completely erased, ppx_globalize generates code that references A.globalize (which does not exist). So we wish to tell ppx_globalize not to "re-globalize" a when deriving B.globalize without leaving the global_ modality in the record.

ppx_globalize already recognizes the [@globalized] attribute for this purpose, so for now we rewrite global_ modalities to the [@globalized] attribute when erasing extensions. Eventually, we will want to come up with a more general approach that all modes and ppxs can share and recognize.

[ccasin]

LGTM (except CI is failing for a reason that looks unrelated to this PR)