When importing a C++ class template instantiation, Swift translates the template parameter type names from C++ into their Swift equivalent.
For instance, `basic_string<wchar_t, char_traits<wchar_t>, allocator<wchar_t>>` gets imported as `basic_string<Scalar, char_traits<Scalar>, allocator<Scalar>>`: `wchar_t` is imported as `CWideChar`, which is a typealias for `Scalar` on most platforms including Darwin. Notice that Swift goes through the `CWideChar` typealias on the specific platform. Another instantiation `basic_string<uint32_t, char_traits<uint32_t>, allocator<uint32_t>>` also gets imported as `basic_string<Scalar, char_traits<Scalar>, allocator<Scalar>>`: `uint32_t` is also imported as `Scalar`. This is problematic because we have two distinct C++ types that have the same name in Swift.
This change makes sure Swift doesn't go through typealiases when emitting names of template parameters, so `wchar_t` would now get printed as `CWideChar`, `int` would get printed as `CInt`, etc.
This also encourages clients to use the correct type (`CInt`, `CWideChar`, etc) instead of relying on platform-specific typealiases.
rdar://115673622
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.
