Unused typedef clang++ warning.

[syclik]

With the new XCode, we are generating more warnings. To see this, run:

> ./runTests.py src/test/unit/lang/reject/reject_func_call_model_test.cpp
...
In file included from src/test/unit/lang/reject/reject_func_call_model_test.cpp:2:
src/test/test-models/good/lang/reject_func_call_model.hpp:26:28: warning: unused typedef
      'fun_return_scalar_t__' [-Wunused-local-typedef]
    typedef fun_scalar_t__ fun_return_scalar_t__;
                           ^
1 warning generated.

One way to fix this is to add -Wno-unused-local-typedef to the clang++ compiler options. Another would be to conditionally generate that typedef, but I think this one might be easier just silencing the new warning.
One fix is to add to clang++ options. Another would be to