Fix an uninitialized variable bug.

In this case it didn't cause any problems, not even non-deterministic behavior, but still it's scary.

include/swift/Basic/ValueEnumerator.h

@@ -22,7 +22,7 @@ namespace swift {
 template<class ValueTy, class IndexTy = size_t>
 class ValueEnumerator {
   /// A running counter to enumerate values.
-  IndexTy counter;
+  IndexTy counter = 0;
 
   /// Maps values to unique integers.
   llvm::DenseMap<ValueTy, IndexTy> ValueToIndex;