re PR c++/5857 (C++ duplicate_decls shouldn't use common_type)

re PR c++/5857 (C++ duplicate_decls shouldn't use common_type)

        PR c++/5857
        * decl.c (duplicate_decls): Use merge_types instead of common_type.
        * typeck.c (common_type): Just hand off to
        type_after_usual_arithmetic_conversions and
        composite_pointer_type.
        (merge_types): New fn.
        (commonparms): Use it instead of common_type.
        (type_after_usual_arithmetic_conversions): Also handle COMPLEX_TYPE.
        (composite_pointer_type): Also handle attributes.
        * cp-tree.h: Declare merge_types.

From-SVN: r50820