[Serialization] Return a nullptr instead of crashing

We got LLDB crash logs where `importer` is dereferenced. In the same crash log
the parent frame correctly checks for nullptr, so this would fix the immediate
crash. In LLDB it is not guaranteed that a ClagnImporter is installed at all
times.

rdar://166224928
(cherry picked from commit 9f59903)

Author: adrian-prantl

lib/Serialization/Deserialization.cpp

@@ -3114,6 +3114,8 @@ ModuleDecl *ModuleFile::getModule(ImportPath::Module name,
     if (!UnderlyingModule && allowLoading) {
       auto importer = getContext().getClangModuleLoader();
       assert(importer && "no way to import shadowed module");
+      if (!importer)
+        return nullptr;
       UnderlyingModule =
           importer->loadModule(SourceLoc(), name.getTopLevelPath());
     }