Missing do_cleanups in ada_make_symbol_completion_list.

Missing do_cleanups in ada_make_symbol_completion_list.

This results in an internal-warning when trying the completion
when in Ada mode:

    (gdb) b simple<TAB>
    /[...]/cleanups.c:265: internal-warning: restore_my_cleanups has found a stale cleanup
    A problem internal to GDB has been detected,
    further debugging may prove unreliable.
    Quit this debugging session? (y or n)

gdb/ChangeLog:

        * ada-lang.c (ada_make_symbol_completion_list): Make sure
        all cleanups are done before returning from this function.

gdb/testsuite/ChangeLog:

        * gdb.ada/complete.exp: Add test verifying completion using
        the "tab" key.