Ask Sema if we enabled modules.

That fixes an issue when running hsimple.C with modules on.

lib/Interpreter/InterpreterCallbacks.cpp

@@ -182,7 +182,7 @@ namespace cling {
     // Disable the ROOT external sema source when we have modules. In the
     // modules case the module manager is taking it's place and we don't want
     // to overwrite it.
-    if (!getenv("ROOT_MODULES") && enableExternalSemaSourceCallbacks)
+    if (!SemaRef.getLangOpts().Modules && enableExternalSemaSourceCallbacks)
       if (!externalSemaSrc || externalSemaSrc == Reader) {
         // If the ExternalSemaSource is the PCH reader we still need to insert
         // our listener.