Code completion: add a lot of infrastructure code

* Added a mode in swift-ide-test to test code completion.  Unlike c-index-test,
  the code completion token in tests is a real token -- we don't need to
  count lines and columns anymore.

* Added support in lexer to produce a code completion token.

* Added a parser interface to code completion.  It is passed down from the
  libFrontend to the parser, but its functions are not called yet.

* Added a sketch of the interface of code completion consumer and code
  completion results.

Note: all this is not doing anything useful yet.


Swift SVN r6128

lib/Frontend/Frontend.cpp

@@ -104,7 +104,8 @@ void swift::CompilerInstance::doIt() {
       parseIntoTranslationUnit(TU, BufferID, &Done, nullptr, &TheParser);
       assert(Done && "Parser returned early?");
       (void) Done;
-      performDelayedParsing(TU, TheParser.get());
+      performDelayedParsing(TU, TheParser.get(),
+                            Invocation->getCodeCompletionFactory());
       TheParser.reset(nullptr);
     }
 
@@ -135,5 +136,7 @@ void swift::CompilerInstance::doIt() {
       performTypeChecking(TU, CurTUElem);
     CurTUElem = TU->Decls.size();
   } while (!Done);
+  performDelayedParsing(TU, TheParser.get(),
+                        Invocation->getCodeCompletionFactory());
 }