DCE: Tasks 5 & 6 - References and CrossFileItems patterns

[cristianoc]

Applies the map → list → merge pattern to references and cross-file items.

Task 5: References

New module: References.ml/.mli

• builder (mutable) for AST processing
• t (immutable) for solver
• Tracks both value refs and type refs
• PosSet for position sets

Changes:

• Thread ~refs:References.builder through AST processing
• addValueReference, addTypeReference use References API
• Solver uses References.find_value_refs, References.find_type_refs
• Deleted global ValueReferences.table and TypeReferences.table

Task 6: CrossFileItems (renamed from DelayedItems)

New module: CrossFileItems.ml/.mli

• builder (mutable) for AST processing
• t (immutable) for processing after merge
• Three item types: exception_refs, optional_arg_calls, function_refs

Changes:

• Thread ~cross_file:CrossFileItems.builder through AST processing
• DeadException.markAsUsed adds to cross_file builder
• DeadOptionalArgs.addReferences, addFunctionReference add to cross_file
• Deleted global delayedItems refs from DeadException and DeadOptionalArgs

Data flow

process_cmt_file (per-file)
    → file_data { annotations; decls; refs; cross_file }

Merge phase:
    FileAnnotations.merge_all → annotations (immutable)
    Declarations.merge_all → decls (immutable)
    CrossFileItems.merge_all → cross_file (immutable)
    References builders merged into refs_builder

Process cross-file items:
    process_exception_refs → writes to refs_builder
    process_optional_args → reads decls

Freeze:
    refs_builder → refs (immutable)

Solver:
    reportDead ~annotations ~decls ~refs

Global state deleted

• DeadCommon.ValueReferences.table
• DeadCommon.TypeReferences.table
• DeadException.delayedItems
• DeadOptionalArgs.delayedItems
• DeadOptionalArgs.functionReferences

Naming

Renamed DelayedItems → CrossFileItems because it better describes the semantic meaning: items that reference across file boundaries.