AST: Fix Type::walk() to visit the count type of a PackExpansionType

Author: slavapestov

lib/AST/TypeWalker.cpp

@@ -57,6 +57,9 @@ class Traversal : public TypeVisitor<Traversal, bool>
   }
 
   bool visitPackExpansionType(PackExpansionType *ty) {
+    if (doIt(ty->getCountType()))
+      return true;
+
     return doIt(ty->getPatternType());
   }