Silence unused variable warning in -Asserts build

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159954 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Serialization/ASTReaderStmt.cpp b/lib/Serialization/ASTReaderStmt.cpp
index 7d56f10..c5325b5 100644
--- a/lib/Serialization/ASTReaderStmt.cpp
+++ b/lib/Serialization/ASTReaderStmt.cpp
@@ -165,6 +165,7 @@
   uint64_t NumAttrs = Record[Idx++];
   AttrVec Attrs;
   Reader.ReadAttributes(F, Attrs, Record, Idx);
+  (void)NumAttrs;
   assert(NumAttrs == S->NumAttrs);
   assert(NumAttrs == Attrs.size());
   std::copy(Attrs.begin(), Attrs.end(), S->Attrs);