[CodeCompletion] Replace main module instead of file

Rather than replacing the code completion file
on the `CompilerInstance` whenever we do a cached
top-level completion, let's set a new main module
instead.

This allows us to properly update the
`LoadedModules` map, and allows the retrieval of
the code completion file to be turned into a
request.

tools/SourceKit/lib/SwiftLang/SwiftCompletion.cpp

@@ -142,8 +142,8 @@ static bool swiftCodeCompleteImpl(
         SwiftConsumer.setContext(&CI.getASTContext(), &CI.getInvocation(),
                                  &CompletionContext);
 
-        auto SF = CI.getCodeCompletionFile();
-        performCodeCompletionSecondPass(*SF.get(), *callbacksFactory);
+        auto *SF = CI.getCodeCompletionFile();
+        performCodeCompletionSecondPass(*SF, *callbacksFactory);
         SwiftConsumer.clearContext();
       });
 }