Teach the frontend how to pass through -Xllvm commands.

This patch allows you to pass through commands to llvm from the driver so now
one can when compiling swift files get debug messages!

Swift SVN r13761

lib/Driver/Tools.cpp

@@ -110,6 +110,9 @@ static void addCommonFrontendArgs(const ToolChain &TC,
   inputArgs.AddAllArgs(arguments, options::OPT_I);
 
   inputArgs.AddLastArg(arguments, options::OPT_g);
+
+  // Pass through any -Xllvm flags.
+  inputArgs.AddAllArgs(arguments, options::OPT_Xllvm);
 }