sorted_vector_map::insert_or_assign() returns the wrong type

A recent commit bea73e236ce14e36bca7cc28e644ff928856adb3 added the `insert_or_assign()` method to `folly::sorted_vector_map` in `folly/sorted_vector_types.h`.

But it is returning an `iterator` instead of `std::pair` as [`std::map` would](https://en.cppreference.com/w/cpp/container/map/insert_or_assign).

Only the overload with a `hint` is supposed to return a plain `iterator`, but that overload wasn't added to `sorted_vector_map` anyway.

Start in folly/sorted_vector_types.h at sorted_vector_map::insert_or_assign(). Compare its return type with the std::map insert_or_assign behavior described in the issue, including the distinction for the hint overload. Done means the existing overload returns the expected pair type and the API matches std::map semantics.

Written by the indexing model from the issue text.