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`.
C++ namespaces are module-independent, but enums are owned by their module's in Swift. So, to prevent declaring two enums with the same name, this patch implements a new approach to namespaces: enums with extensions.
Here's an example:
```
// Module A
namespace N { void test1(); }
// Module B
namespace N { void test2(); }
// __ObjC module
enum N { }
// Swift module A
extension N { func test1() }
// Swift module B
extension N { func test1() }
```
Thanks to @gribozavr for the great idea.
