Introduce -fno-debug-types-section flag.

Introduce -fno-debug-types-section flag.

        * common.opt (fdebug-types-section): New flag.
        * doc/invoke.texi: Document new -fno-debug-types-section flag.
        * dwarf2out.c (use_debug_types): New define.
        (struct die_struct): Mark die_id with GTY desc use_debug_types.
        (print_die): Guard output of type unit signatures using
        use_debug_types.
        (build_abbrev_table): Replace assert of dwarf_version >= 4
        with assert on use_debug_types.
        (size_of_die): Likewise.
        (unmark_dies): Likewise.
        (value_format): Decide AT_ref_external form on use_debug_types.
        (output_die): Replace dwarf_version version check guard with
        use_debug_types where appropriate.
        (modified_type_die): Likewise.
        (gen_reference_type_die): Likewise.
        (dwarf2out_start_source_file): Likewise.
        (dwarf2out_end_source_file): Likewise.
        (prune_unused_types_walk_attribs): Likewise.
        (dwarf2out_finish): Likewise.

From-SVN: r171742