Fix the build, because we still need the ret_shorty.

Also, changed LOG(ERROR) to LOG(FATAL) and added "TODO", to address
bdc's comments on the last commit. Forgot to --amend earlier, so it's a
separate commit here.

Change-Id: I3edf1e652e5c6b4da1251cab9219b8b2b3572203
diff --git a/src/compiler_llvm/upcall_compiler.cc b/src/compiler_llvm/upcall_compiler.cc
index 308790a..8dc4e98 100644
--- a/src/compiler_llvm/upcall_compiler.cc
+++ b/src/compiler_llvm/upcall_compiler.cc
@@ -148,7 +148,7 @@
       args.push_back(irb_.CreateLoad(arg_addr));
 
     } else {
-      LOG(ERROR) << "Unexpected arg shorty for invoke stub: " << shorty[i];
+      LOG(FATAL) << "Unexpected arg shorty for invoke stub: " << shorty[i];
     }
   }