Use '(void)' in Metadata.cpp to avoid "variable unused" warning in non-assert builds.

Author: kubamracek

stdlib/public/runtime/Metadata.cpp

@@ -2741,6 +2741,7 @@ allocateWitnessTable(GenericWitnessTable *genericTable,
   // default implementations.
   size_t numMandatoryRequirements = protocol->NumMandatoryRequirements;
   assert(numPatternWitnesses >= numMandatoryRequirements);
+  (void)numMandatoryRequirements;
 
   // The total number of requirements.
   size_t numRequirements = protocol->NumRequirements;