fix SourceKit injected VFS with .swiftinterface files

Use `.setFileSystem()` instead of new `setup` method.

tools/SourceKit/lib/SwiftLang/SwiftCompletion.cpp

@@ -197,7 +197,8 @@ static bool swiftCodeCompleteImpl(
   // FIXME: We need to be passing the buffers from the open documents.
   // It is not a huge problem in practice because Xcode auto-saves constantly.
 
-  if (CI.setup(Invocation, FileSystem)) {
+  CI.getSourceMgr().setFileSystem(FileSystem);
+  if (CI.setup(Invocation)) {
     // FIXME: error?
     return true;
   }