[Driver] Use Twine instead of itostr. NFC.

No need to construct temporary std::strings here.

llvm-svn: 249676
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 83b5869..0738f13 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -2353,7 +2353,7 @@
   }
   if (DwarfVersion > 0)
     CmdArgs.push_back(
-        Args.MakeArgString("-dwarf-version=" + llvm::itostr(DwarfVersion)));
+        Args.MakeArgString("-dwarf-version=" + Twine(DwarfVersion)));
 }
 
 static void CollectArgsForIntegratedAssembler(Compilation &C,