[BatchMode] Avoid spurious warnings in sourcekitd and indexing

Explicitly disable batch mode in createCompilerInvocation, since it uses
-force-single-frontend-invocation.  Previously we were getting spurious
warnings.  Also add a test that -disable-batch-mode will allow commands
that use -index-file to avoid the same warning, since that is likely
what they want to do as well.

rdar://39581506

lib/Driver/FrontendUtil.cpp

@@ -34,6 +34,11 @@ swift::driver::createCompilerInvocation(ArrayRef<const char *> Argv,
   // frontend command.
   Args.push_back("-force-single-frontend-invocation");
 
+  // Explictly disable batch mode to avoid a spurious warning when combining
+  // -enable-batch-mode with -force-single-frontend-invocation.  This is an
+  // implementation detail.
+  Args.push_back("-disable-batch-mode");
+
   // Avoid using filelists
   std::string neverThreshold =
       std::to_string(Compilation::NEVER_USE_FILELIST);