Fix a missing lambda return type that tripped the builders

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@334166 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/PPCGCodeGeneration.cpp b/lib/CodeGen/PPCGCodeGeneration.cpp
index acc8ef0..40886de 100644
--- a/lib/CodeGen/PPCGCodeGeneration.cpp
+++ b/lib/CodeGen/PPCGCodeGeneration.cpp
@@ -1408,7 +1408,7 @@
 ///
 /// Return "" if we are not compiling for CUDA.
 std::string getCUDALibDeviceFuntion(Function *F) {
-  const std::string FnName = [&] {
+  auto FnName = [&] () -> const std::string {
     auto It = IntrinsicToLibdeviceFunc.find(F->getName());
     if (It != IntrinsicToLibdeviceFunc.end())
       return It->second;