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/Frontend/CompilerInvocation.cpp

@@ -455,6 +455,7 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
 
   Opts.EnableSourceImport = Args.hasArg(OPT_enable_source_import);
   Opts.SILSerializeAll = Args.hasArg(OPT_sil_serialize_all);
+  Opts.LLVMArgs = Args.getAllArgValues(OPT_Xllvm);
 
   return false;
 }