fortran/

fortran/
	PR fortran/54730
	* gfortran.h (struct gfc_undo_change_set): New field 'previous'.
	(gfc_new_undo_checkpoint, gfc_drop_last_undo_checkpoint,
	gfc_restore_last_undo_checkpoint): New prototypes.
	* symbol.c (default_undo_chgset_var): Update initialization.
	(single_undo_checkpoint_p, gfc_new_undo_checkpoint,
	free_undo_change_set_data, pop_undo_change_set,
	gfc_drop_last_undo_checkpoint, enforce_single_undo_checkpoint):
	New functions.
	(save_symbol_data): Handle multiple change sets.  Make sure old_symbol
	field's previous value is not overwritten.  Clear gfc_new field.
	(restore_old_symbol): Restore previous old_symbol field.
	(gfc_restore_last_undo_checkpoint): New function, using body renamed
	from gfc_undo_symbols.  Restore the previous change set as current one.
	(gfc_undo_symbols): New body.
	(gfc_commit_symbols, gfc_commit_symbol, gfc_enforce_clean_symbol_state):
	Call enforce_single_undo_checkpoint.
	(gfc_symbol_done_2): Ditto.  Free change set data.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@196414 138bc75d-0d04-0410-961f-82ee72b054a4