[llvm][unittests] Silence warning when building with MSVC
Fixes:
```
[836/1441] Building CXX object unittests\ExecutionEngine\Orc\CMakeFiles\OrcJITTests.dir\CallableTraitsHelperTest.cpp.obj
C:\git\llvm-project\llvm\unittests\ExecutionEngine\Orc\CallableTraitsHelperTest.cpp(25): warning C4551: function call missing argument list
C:\git\llvm-project\llvm\unittests\ExecutionEngine\Orc\CallableTraitsHelperTest.cpp(34): warning C4551: function call missing argument list
```