Remove more superfluous .str() and replace std::string concatenation with Twine.

Following r233392, http://llvm.org/viewvc/llvm-project?rev=233392&view=rev.

llvm-svn: 233555
diff --git a/llvm/lib/LTO/LTOModule.cpp b/llvm/lib/LTO/LTOModule.cpp
index 49aa97d..5cdbca6 100644
--- a/llvm/lib/LTO/LTOModule.cpp
+++ b/llvm/lib/LTO/LTOModule.cpp
@@ -267,7 +267,7 @@
       Constant *cn = gvn->getInitializer();
       if (ConstantDataArray *ca = dyn_cast<ConstantDataArray>(cn)) {
         if (ca->isCString()) {
-          name = ".objc_class_name_" + ca->getAsCString().str();
+          name = (".objc_class_name_" + ca->getAsCString()).str();
           return true;
         }
       }