Function gc_node_at

src/ast/ast_gc_report.cpp:17–28

---- per-tag downcast ---- Only the AST layer knows the concrete subclass behind each gc tag, so this is the one place that can recover a node's source `at`. MakeStruct (and any untagged node) carries no `at` of its own -> nullptr.

    // one place that can recover a node's source `at`. MakeStruct (and any untagged node)
    // carries no `at` of its own -> nullptr.
    static const LineInfo * gc_node_at ( const gc_node * node ) {
        switch ( node->gc_type_tag() ) {
            case GC_TAG_TYPEDECL:       return &static_cast<const TypeDecl*>(node)->at;
            case GC_TAG_EXPRESSION:     return &static_cast<const Expression*>(node)->at;
            case GC_TAG_MAKEFIELDDECL:  return &static_cast<const MakeFieldDecl*>(node)->at;
            case GC_TAG_FUNCTION:       return &static_cast<const Function*>(node)->at;
            case GC_TAG_ENUMERATION:    return &static_cast<const Enumeration*>(node)->at;
            case GC_TAG_STRUCTURE:      return &static_cast<const Structure*>(node)->at;
            case GC_TAG_VARIABLE:       return &static_cast<const Variable*>(node)->at;
            default:                    return nullptr;     // MakeStruct, untagged, builtins
        }
    }

    static const char * gc_tag_name ( uint16_t tag ) {
        switch ( tag ) {