[CodeCompletion] Avoid typechecking all toplevel decls in the current file

- Use `performParseAndResolveImportsOnly()` to invoke the frontend
- Do `bindExtensions()` in `ide::typeCheckContextUntil()`
- Typecheck preceding `TopLevelCodeDecl`s only if the compleiton is in
  a `TopLevelCodeDecl`
- Other related tweaks

rdar://problem/56636747

tools/SourceKit/lib/SwiftLang/SwiftCompletion.cpp

@@ -212,7 +212,7 @@ static bool swiftCodeCompleteImpl(
   SwiftConsumer.setContext(&CI.getASTContext(), &Invocation,
                            &CompletionContext);
   registerIDETypeCheckRequestFunctions(CI.getASTContext().evaluator);
-  CI.performSema();
+  CI.performParseAndResolveImportsOnly();
   SwiftConsumer.clearContext();
 
   return true;