`std::set::insert` isn't exposed into Swift, because it returns an instance of an unsafe type.
This change adds a Swift overload of `insert` for `std::set` and `std::unordered_set` that has the return type identical to `Swift.Set.insert`: a tuple of `(inserted: Bool, memberAfterInsert: Element)`.
rdar://111036912
This adds a protocol to the C++ standard library overlay which will improve the ergonomics of `std::set`, `std::unordered_set` and `std::multiset` when used from Swift code.
As of now, `CxxSet` adds a `contains` function to C++ sets.
C++ stdlib set types are automatically conformed to `CxxSet`: `std::set`, `unordered_set`, `std::multiset`. Custom user types are not conformed to `CxxSet` automatically: while a custom type might have an interface similar to `std::set`, the semantics might differ, and adding a conformance would cause confusion.
