Don't set TUScope to null when generating a module in incremental processing mode.

[teemperor]

Summary

When in incremental processing mode, we should never set TUScope to a nullptr otherwise any future lookups fail. We already have similar checks in the rest of the code, but we never hit this one because so far we didn't try to generate a module from the AST that Cling generates.

[v.g.vassilev]

I think we can rely on a post-commit review here. LGTM!

lib/Sema/Sema.cpp

if (Module *CurrentModule = PP.getCurrentModule()) {
       }
     }
 
     // Warnings emitted in ActOnEndOfTranslationUnit() should be emitted for
     // modules when they are built, not every time they are used.
     emitAndClearUnusedLocalTypedefWarnings();
 
     // Modules don't need any of the checking below.
+    if (!PP.isIncrementalProcessingEnabled())
       TUScope = nullptr;
     return;
   }
 
   // C99 6.9.2p2:
   //   A declaration of an identifier for an object that has file
   //   scope without an initializer, and without a storage-class
   //   specifier or with the storage-class specifier static,
   //   constitutes a tentative definition. If a translation unit