add a method to improve compatibility with SmallVector.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146928 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/SetVector.h

@@ -144,6 +144,12 @@ public:
     set_.erase(back());
     vector_.pop_back();
   }
+  
+  T pop_back_val() {
+    T Ret = back();
+    pop_back();
+    return Ret;
+  }
 
   bool operator==(const SetVector &that) const {
     return vector_ == that.vector_;