Put an unused result attribute on SmallSet::empty()

This matches other empty() container functions in LLVM.

No actual usage problems discovered in this instance.

llvm-svn: 195562
diff --git a/llvm/include/llvm/ADT/SmallSet.h b/llvm/include/llvm/ADT/SmallSet.h
index 5dfe924..ecd3843 100644
--- a/llvm/include/llvm/ADT/SmallSet.h
+++ b/llvm/include/llvm/ADT/SmallSet.h
@@ -39,7 +39,10 @@
 public:
   SmallSet() {}
 
-  bool empty() const { return Vector.empty() && Set.empty(); }
+  bool LLVM_ATTRIBUTE_UNUSED_RESULT empty() const {
+    return Vector.empty() && Set.empty();
+  }
+
   unsigned size() const {
     return isSmall() ? Vector.size() : Set.size();
   }