[OpenMP] Fix second debug name from map clause

This patch fixes a bug from D89802.  For example, without it, Clang
generates x as the debug map name for both x and y in the following
example:

```
 #pragma omp target map(to: x, y)
 x = y = 1;
```

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D101564