[clang][OpenMP][DebugInfo] Debug support for variables in shared clause of OpenMP task construct

Currently variables appearing inside shared clause of OpenMP task construct
are not visible inside lldb debugger.

After the current patch, lldb is able to show the variable

```
* thread #1, name = 'a.out', stop reason = breakpoint 1.1
    frame #0: 0x0000000000400934 a.out`.omp_task_entry. [inlined] .omp_outlined.(.global_tid.=0, .part_id.=0x000000000071f0d0, .privates.=0x000000000071f0e8, .copy_fn.=(a.out`.omp_task_privates_map. at testshared.cxx:8), .task_t.=0x000000000071f0c0, __context=0x000000000071f0f0) at testshared.cxx:10:34
   7      else {
   8    #pragma omp task shared(svar) firstprivate(n)
   9        {
-> 10         printf("Task svar = %d\n", svar);
   11         printf("Task n = %d\n", n);
   12         svar = fib(n - 1);
   13       }
(lldb) p svar
(int) $0 = 9
```

Reviewed By: djtodoro

Differential Revision: https://reviews.llvm.org/D115510