Replace use of SmallVector::back + pop_back with pop_back_val

I ran across an instance where the value was being loaded
out via back, then immediately popped.  Since pop_back_val
is more efficient at this (it moves out), replace this 
instance.

llvm-svn: 316015
diff --git a/clang/lib/Sema/AnalysisBasedWarnings.cpp b/clang/lib/Sema/AnalysisBasedWarnings.cpp
index 08fc080..bdfed6e 100644
--- a/clang/lib/Sema/AnalysisBasedWarnings.cpp
+++ b/clang/lib/Sema/AnalysisBasedWarnings.cpp
@@ -361,8 +361,7 @@
   SmallVector<CFGBlock *, 16> Stack;
   Stack.push_back(&BodyCFG->getEntry());
   while (!Stack.empty()) {
-    CFGBlock *CurBlock = Stack.back();
-    Stack.pop_back();
+    CFGBlock *CurBlock = Stack.pop_back_val();
 
     unsigned ID = CurBlock->getBlockID();
     ThrowState CurState = States[ID];