[3.24.0] backport #14715

[Alizter]

Backport #14715 onto 3.24.0-rc.

Fixes the melange build failure on OCaml 4.14 where dune emits an I/O error: jscomp/runtime/.melange_src/X.mli: No such file or directory against every melange-re/melange.*-414 reverse dep (see ocaml/opam-repository#29996 — the 3.24.0~alpha2 release PR). The fix is to skip the line directive in setup_melange_sources_copy_rules when a module uses non-staged PPX, since the PPX pipeline now passes -loc-filename separately and adding the directive would double-shift diagnostics.

The fix has a small precursor chain on main; bringing the headline #14715 alone doesn't apply cleanly because subsequent PRs introduced the test files and machinery the fix's diff targets. Included PRs (chronological):

• test(melange): show single context melange loses ppx snippet previews #14697 — test: show single context melange loses ppx snippet previews
• test(melange): show single context reason+melange loses ppx snippet p… #14698 — test: show single context reason+melange loses ppx snippet previews
• test(melange): accept ppx loc-filename in fixtures #14708 — test: accept ppx loc-filename in fixtures
• fix(melange): preserve original ppx source locations #14706 — fix: preserve original ppx source locations (introduces -loc-filename)
• test(melange): reproduce reason source location regression #14714 — test: reproduce reason source location regression
• fix(melange): avoid line directive when passing -loc-filename #14715 — fix: avoid line directive when passing -loc-filename

[Alizter]

@anmonteiro could you take a look to see if this makes sense to backport like this?

[shonfeder]

Discussed earlier today, we can move forward with merging the backport, but would still welcome consideration by @anmonteiro when they have time.