[cxx-interop] Allow import-as-member for types in namespaces
This adds support for `swift_name` attribute being used with C++ types that are declared within namespaces, e.g.
```
__attribute__((swift_name("MyNamespace.MyType.my_method()")))
```
Previously import-as-member would only accept a top-level unqualified type name.
rdar://138934888
