[clang] [clang][OpenMP] Support for reduction clause with array elements as modifier (PR #160846)

[via cfe-commits]

SunilKuravinakop wrote:

We found in our testing that in code like:

```
#pragma omp parallel num_threads(4)
#pragma omp for reduction (+:a[1])
for   (i = 0; i < 10; i++)

{     a[1] += i; }
```
reduction was not being done properly. Hence this fix.

https://github.com/llvm/llvm-project/pull/160846