* tree.c (struct free_lang_data_d): Add constructor and change
types of members to ones that automatically manage resources.
(fld_worklist_push): Adjust.
(find_decls_types): Likewise.
(find_decls_types_in_eh_region): Likewise.
(free_lang_data_in_cgraph): Stop manually creating and
destroying members of free_lang_data_d.