[libomptarget][nfc] Improve static assert message in dlwrap

Revision of D102858. Raise dlwrap arity argument to template argument
so the correct value is given in the error message. E.g. '2 == 1' instead of
'2 == trait<>::nargs'.

Arity higher than it should be:
Before diff
```
$/plugins/cuda/dynamic_cuda/cuda.cpp:23:1: error:
      static_assert failed due to requirement '2 == trait<cudaError_enum (*)(unsigned int)>::nargs'
      "Arity Error"
DLWRAP_INTERNAL(cuInit, 2);
^~~~~~~~~~~~~~~~~~~~~~~~~~
...
$/include/dlwrap.h:166:3: note: expanded from macro
      'DLWRAP_COMMON'
  static_assert(ARITY == trait<decltype(&SYMBOL)>::nargs, "Arity Error");      \
```

After diff
In file included from $/plugins/cuda/dynamic_cuda/cuda.cpp:16:
```
$/include/dlwrap.h:131:3: error: static_assert failed due to
      requirement '2UL == 1UL' "Arity Error"
  static_assert(Requested == Required, "Arity Error");
  ^             ~~~~~~~~~~~~~~~~~~~~~
$/plugins/cuda/dynamic_cuda/cuda.cpp:23:1: note: in
      instantiation of function template specialization 'dlwrap::verboseAssert<2UL, 1UL>' requested
      here
DLWRAP_INTERNAL(cuInit, 2);
```

Arity lower than it should be:
Before diff
```
$/plugins/cuda/dynamic_cuda/cuda.cpp:131:10: error: no
      matching function for call to 'dlwrap_cuInit'
  return dlwrap_cuInit(X);
         ^~~~~~~~~~~~~
$/plugins/cuda/dynamic_cuda/cuda.cpp:23:1: note: candidate
      function not viable: requires 0 arguments, but 1 was provided
DLWRAP_INTERNAL(cuInit, 0);
```

After diff
In file included from $/plugins/cuda/dynamic_cuda/cuda.cpp:16:
```
$/include/dlwrap.h:131:3: error: static_assert failed due to
      requirement '0UL == 1UL' "Arity Error"
  static_assert(Requested == Required, "Arity Error");
  ^             ~~~~~~~~~~~~~~~~~~~~~
$/plugins/cuda/dynamic_cuda/cuda.cpp:23:1: note: in
      instantiation of function template specialization 'dlwrap::verboseAssert<0UL, 1UL>' requested
      here
DLWRAP_INTERNAL(cuInit, 0);
```

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106543