[Libomptarget] Don't report lack of CUDA devices

Sometimes libomptarget's CUDA plugin produces unhelpful diagnostics
about a lack of CUDA devices before an application runs:

```
$ clang -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa hello-world.c
$ ./a.out
CUDA error: Error returned from cuInit
CUDA error: no CUDA-capable device is detected
Hello World: 4
```

This can happen when the CUDA plugin was built but all CUDA devices
are currently disabled in some manner, perhaps because
`CUDA_VISIBLE_DEVICES` is set to the empty string.  As shown in the
above example, it can even happen when we haven't compiled the
application for offloading to CUDA.

The following code from `openmp/libomptarget/plugins/cuda/src/rtl.cpp`
appears to be intended to handle this case, and it chooses not to
write a diagnostic to stderr unless debugging is enabled:

```
if (NumberOfDevices == 0) {
  DP("There are no devices supporting CUDA.\n");
  return;
}
```

The problem is that the above code is never reached because the
earlier `cuInit` returns `CUDA_ERROR_NO_DEVICE`.  This patch handles
that `cuInit` case in the same manner as the above code handles the
`NumberOfDevices == 0` case.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D130371