Fix DCE false positives on ReScript compiler CMT analysis

[JonoPrest]

This fixes several DCE false positives found while running reanalyze -dce-cmt _build/default against the ReScript compiler after dune build @check.

The issues were mostly around CMT-based analysis losing liveness information in compiler-shaped code:

• fallback to cmt_sourcefile when typedtree locations point at missing generated/preprocessed files, like *.cppo.ml
• keep parent modules live when nested module values are live
• mark values from functor argument signatures as referenced, e.g. Set.Make(Ordered).compare
• keep side-effectful local lets live from their enclosing binding

Each fix is paired with a regression test that fails on ocaml-5-3 and passes with the fix.

[JonoPrest]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Restrict functor refs to required signature

When a functor is applied to a module whose inferred signature contains values beyond the functor parameter signature, this walks argumentExpr.mod_type and adds references to every value in the actual argument module. For example, module Arg = struct type t = int let compare = ... let unused = ... end; module S = Set.Make(Arg) will keep Arg.unused live even though the functor cannot access it, causing DCE to miss genuinely dead values in functor argument modules.

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