Trust PAD types instead of using PAD___XVS.

    Trust PAD types instead of using PAD___XVS.
        * ada-lang.c (trust_pad_over_xvs): New static variable.
        (ada_is_aligner_type): If !trust_pad_over_xvs and there is a
        parallel XVS type, follow the XVS type instead of the PAD type.
        (unwrap_value): Make sure that there is no parallel XVE type
        before returning the value as is.
        (set_ada_list, show_ada_list): New static variables.
        (set_ada_command, show_ada_command): New functions.
        (_initialize_ada_language): Add new "set/show ada" prefix commands.
        Add new "set/show ada trust-PAD-over-XVS" setting.