Ambiguity on `Symbol::For` overload between `std::string` and `std::string_view` convertible

[legendecas]

#1722 introduced overloads on Symbol::For with std::string_view, but this also introduced an ambiguity when a type is convertible to both std::string and std::string_view.

An alternative path forward is that we should remove std::string overload when introducing std::string_view support when applicable.