Swift: Add basic support for experimental using decls

Note that we cannot write tests for these at the moment. Passing
```
-enable-experimental-feature DefaultIsolationPerFile
```
to the extractor results in:
```
error: experimental feature 'DefaultIsolationPerFile' cannot be enabled in production compile
```

swift/schema.py

@@ -117,6 +117,10 @@ class GenericContext(Element):
 class EnumCaseDecl(Decl):
     elements: list["EnumElementDecl"]
 
+class UsingDecl(Decl):
+    is_main_actor: predicate
+    is_nonisolated: predicate
+
 class ExtensionDecl(GenericContext, Decl):
     extended_type_decl: "NominalTypeDecl"
     protocols: list["ProtocolDecl"]