add warning for repeated use of weak refs

Author: diederich

Base/Common.xcconfig

@@ -47,6 +47,9 @@ CLANG_WARN_IMPLICIT_SIGN_CONVERSION = NO
 // For example, this can catch issues when one incorrectly intermixes using NSNumbers and raw integers.
 CLANG_WARN_INT_CONVERSION = YES;
 
+// Warn about repeatedly using a weak reference without assigning the weak reference to a strong reference.
+CLANG_WARN_OBJC_REPEATED_USE_OF_WEAK = YES
+
 // Warn about classes that unintentionally do not subclass a root class (such as NSObject).
 CLANG_WARN_OBJC_ROOT_CLASS = YES