provide a ui_out destructor.
* ui-out.h: Declare the new ui_out destructor.
(ui_out_impl): Add a field for data destructor in ui_out_impl.
* ui-out.c (default_data_destroy): Add a default data destructor
which does nothing.
(default_ui_out_impl): Set the new data_destroy field to
default_data_destroy
(uo_data_destroy): Local function which invokes the data
destructor if present.
(clear_table): Local function which clears the table data of a
ui_out object.
(ui_out_destroy): Public function which frees a ui_out object.
(ui_out_table_end): Use the new clear_table function.
* cli-out.c (cli_ui_out_impl): Set the new data_destroy field to
NULL.
* mi/mi-out.c (mi_ui_out_impl): Set the new data_destroy field
to NULL.
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