ART: Mask garbage-memory warnings

Add invariants or NOLINT to let clang-analyzer understand that
there's no garbage involved.

(The analysis itself is too expensive to force it on for all
 local builds.)

Test: mmma art
Change-Id: I6148d743321248195abb3a8f646e63941f11f9a9
diff --git a/runtime/class_loader_utils.h b/runtime/class_loader_utils.h
index af42878..78ad568 100644
--- a/runtime/class_loader_utils.h
+++ b/runtime/class_loader_utils.h
@@ -147,8 +147,14 @@
                                      Handle<mirror::ClassLoader> class_loader,
                                      Visitor fn)
     REQUIRES_SHARED(Locks::mutator_lock_) {
-  auto helper = [&fn](const art::DexFile* dex_file, void** ATTRIBUTE_UNUSED)
+  auto helper = [&fn](const art::DexFile* dex_file, void** ret)
       REQUIRES_SHARED(Locks::mutator_lock_) {
+#ifdef __clang_analyzer__
+    *ret = nullptr;
+#else
+    UNUSED(ret);
+#endif
+
     return fn(dex_file);
   };
   VisitClassLoaderDexFiles<decltype(helper), void*>(soa,