[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 #10 from Yu Xuanchi <yuxuanchiadm at 126 dot com> ---
So there is currently i'm working on:
-Add "OVERLOAD" tree or just use chain. So we can put all function with same
identifier overloaded in a same tree so we can obtain them use
build_external_ref.
-Find a way to handle function invoke overload resolution.
-Find a way to handle function address-of overload resolution.