[Bug libstdc++/83328] string.insert does not return a iterator when using initializer lists

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83328

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixing it is an ABI change, so it's not that simple.

It should also have a const_iterator parameter, not iterator, and needs to be
fixed for both std::string implementations.