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/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
index 7ff7f9a..0587335 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
@@ -255,7 +255,7 @@
   std::string ArchFS = ARM_MC::ParseARMTriple(TT, CPU);
   if (!FS.empty()) {
     if (!ArchFS.empty())
-      ArchFS = ArchFS + "," + FS.str();
+      ArchFS = (Twine(ArchFS) + "," + FS).str();
     else
       ArchFS = FS;
   }