Avoid int to string conversion in Twine or raw_ostream contexts.

Some output changes from uppercase hex to lowercase hex, no other functionality change intended.

llvm-svn: 321526
diff --git a/clang/lib/Driver/ToolChains/Darwin.cpp b/clang/lib/Driver/ToolChains/Darwin.cpp
index 289f4ed..2250e82 100644
--- a/clang/lib/Driver/ToolChains/Darwin.cpp
+++ b/clang/lib/Driver/ToolChains/Darwin.cpp
@@ -545,8 +545,7 @@
   if (unsigned Parallelism =
           getLTOParallelism(Args, getToolChain().getDriver())) {
     CmdArgs.push_back("-mllvm");
-    CmdArgs.push_back(
-        Args.MakeArgString(Twine("-threads=") + llvm::to_string(Parallelism)));
+    CmdArgs.push_back(Args.MakeArgString("-threads=" + Twine(Parallelism)));
   }
 
   if (getToolChain().ShouldLinkCXXStdlib(Args))