Clang is using C++17 standard version by default since Clang 16.
Swift’s ClangImporter should do the same, to make sure that clients who run clang and then swiftc without explicit std version see consistent behavior.
rdar://125777068
This makes sure that `std::function` is imported consistently on supported platforms, and that it allows basic usage: calling a function with `callAsFunction`, initializing an empty function, and passing a function retrieved from C++ back to C++ as a parameter.
rdar://103979602
