[Swift-ObjC-Interop] Speculative cyclic lookup fix

The lookup to resolve an ObjC forward declaration
to its potential native Swift definition within
a mixed module is becoming cyclic somehow.

The old uncached lookup still works, so it seems
this is an issue with cache invalidation somehow.
Until the route of the issue, never mark the cache
as populated. This is the same behaviour
as before the cached lookup was introduced, but
we can easily turn the caching back on later.

Author: Nuri Amari

lib/AST/Module.cpp

@@ -1326,7 +1326,9 @@ void ModuleDecl::populateObjCNameLookupCache() {
     }
   }
 
-  setIsObjCNameLookupCachePopulated(true);
+  // TODO: Caching these lookups is causing cyclic lookups, renable
+  // the caching once the issue is identified
+  /* setIsObjCNameLookupCachePopulated(true); */
 }
 
 void SourceFile::getTopLevelDecls(SmallVectorImpl<Decl*> &Results) const {