[Bug c/71199] Support overloadable attribute in GNU C front-end

[yuxuanchiadm at 126 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71199

--- Comment #8 from Yu Xuanchi <yuxuanchiadm at 126 dot com> ---
I have test extern "C" in clang. Look like clang dose not actually mangled it
to "C format". I think because clang C front-end actually not have any mangle
logic. So it just mangled it like CPP front-end does.