[merlin] preprocess + run results in -pp and not -ppx #4169
Description
It seems that while ocamlc is happy to receive marshaled AST, ocamllsp(merlin) is not happy receiving mashaled AST if a -pp is given (a -ppx would work).
At the same time preprocess with a user action generates a merlin config which passes -pp, and there seem to be no way to override this decision.
Expected Behavior
(preprocess (per_module ((action (run "bla")) m)))
should/could generate a -ppx directive for merlin
Actual Behavior
generates always a -pp directive for merlin
Reproduction
opam list | grep serv
# ocaml-lsp-server 1.4.0 LSP Server for OCaml
opam list | grep dune
# dune 2.8.2 Fast, portable, and opinionated build system
# dune-build-info 2.8.2 Embed build informations inside executable
# dune-configurator 2.8.2 Helper library for gathering system configuration
# dune-private-libs 2.6.2 Private libraries of Dune
git clone https://github.com/LPCIC/elpi
git checkout rm-merlin-hack
dune build
code . # open src/data.ml or src/runtime_trace_on.ml