[cxx-interop] Import non-member operators as global functions

[egorzhdan]

If a non-member operator is declared in a C++ namespace, we previously imported it as a static member of the enum that represents the C++ namespace.

This is not always correct under Swift rules for operators. In pure Swift, this code is valid:

public protocol UnsafeCxxRandomAccessIterator {
  static func +=(lhs: inout Self, rhs: Int)
}

enum std {
  public struct A : UnsafeCxxRandomAccessIterator {
    public static func += (lhs: inout A, rhs: Int) {
    }
  }
}

but this is not valid:

public protocol UnsafeCxxRandomAccessIterator {
  static func +=(lhs: inout Self, rhs: Int)
}

enum std {
  public struct A : UnsafeCxxRandomAccessIterator {}
  public static func += (lhs: inout A, rhs: Int) {}
}
// error: Member operator '+=' must have at least one argument of type 'std'

This caused assertion failures in SILGen when conforming C++ iterator types to UnsafeCxxRandomAccessIterator.

[egorzhdan]

@swift-ci please smoke test

[egorzhdan]

@swift-ci please smoke test

[zoecarver]

LGTM thanks