Previously a namespace declaration was imported along with all of its redeclarations, and their members were added to a single Swift extension. This was problematic when a single namespace is declared in multiple modules – the extension belonged to only one of them.
For an example of this, try printing a module interface for `std.string`/`std.iosfwd` – it will be empty, even though the declarations from those modules are actually imported into Swift correctly.
This change makes sure that when we're importing different redeclarations of the same namespace, we're adding them as separate extensions to appropriate modules.
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.
