[OpenMP] Fix map clause for unused var: don't ignore it

For example, without this patch:

```
 $ cat test.c
 int main() {
   int x[3];
   #pragma omp target map(tofrom:x[0:3])
 #ifdef USE
   x[0] = 1
 #endif
   ;
   return 0;
 }
 $ clang -fopenmp -fopenmp-targets=nvptx64-nvidia-cuda -S -emit-llvm test.c
 $ grep '^@.offload_maptypes' test.ll
 $ echo $?
 1
 $ clang -fopenmp -fopenmp-targets=nvptx64-nvidia-cuda -S -emit-llvm test.c \
         -DUSE
 $ grep '^@.offload_maptypes' test.ll
 @.offload_maptypes = private unnamed_addr constant [1 x i64] [i64 35]
```

With this patch, both greps produce the same result.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D83922