Ensure correct file position attributes for group use prefix
-----
<?php
use Foo\Bar\{Baz};
-----
!!positions
array(
    0: Stmt_GroupUse[2:1 - 2:17](
        type: TYPE_UNKNOWN (0)
        prefix: Name[2:5 - 2:11](
            parts: array(
                0: Foo
                1: Bar
            )
        )
        uses: array(
            0: Stmt_UseUse[2:14 - 2:16](
                type: TYPE_NORMAL (1)
                name: Name[2:14 - 2:16](
                    parts: array(
                        0: Baz
                    )
                )
                alias: Baz
            )
        )
    )
)