You can retain foreign key constraints by having one column per type of reference. It is also possible to ensure that exactly one column of several is NOT NULL, so that the columns can always be mapped to an enum in application code. Also, in PostgreSQL, the storage for the extra NULLs uses just one bit per column in a bitmap.

    CREATE TABLE dirents (
        parent         bigint   NOT NULL,
        child_dir      bigint,
        child_file     bigint,
        child_symlink  bigint,
        basename       text     NOT NULL,
    
        -- Ensure exactly one type of child is set
        CHECK (num_nonnulls(child_dir, child_file, child_symlink) = 1),
    
        CONSTRAINT dirents_child_dir_fkey     FOREIGN KEY (child_dir)     REFERENCES dirs (id),
        CONSTRAINT dirents_child_file_fkey    FOREIGN KEY (child_file)    REFERENCES files (id),
        CONSTRAINT dirents_child_symlink_fkey FOREIGN KEY (child_symlink) REFERENCES symlinks (id),
    
        PRIMARY KEY (parent, basename)
    );