[llvm] Add assertions for the smart pointers with the possibility to be null in ModuleLazyLoaderCache::operator()

Split from D91844.

The return value of function `ModuleLazyLoaderCache::operator()` in file llvm/tools/llvm-link/llvm-link.cpp. According to the bug report of my static analyzer, the std::function variable `ModuleLazyLoaderCache::createLazyModule` points to function `loadFile`, which may return `nullptr` when error. And the pointer is dereferenced without a check.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D97258

tools/llvm-link/llvm-link.cpp

@@ -246,8 +246,10 @@ public:
 Module &ModuleLazyLoaderCache::operator()(const char *argv0,
                                           const std::string &Identifier) {
   auto &Module = ModuleMap[Identifier];
-  if (!Module)
+  if (!Module) {
     Module = createLazyModule(argv0, Identifier);
+    assert(Module && "Failed to create lazy module!");
+  }
   return *Module;
 }
 } // anonymous namespace