Use double-checked locking for @memoize methods.

Throw NPE if a not-nullable @memoize method returns null.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123875665

factory/src/it/functional/src/main/java/com/google/auto/factory/FactoryGenerated.java

@@ -39,6 +39,7 @@ public final class FactoryGenerated {
   }
 
   // Generates second factory method with a different name for the Dependency dependency.
+  // Tests http://b/21632171.
   FactoryGenerated(
       Object name,
       @Provided Dependency dependency2,