[clang][OpenMP][DebugInfo] Debug support for private variables inside an OpenMP task construct

Currently variables appearing inside private/firstprivate/lastprivate
clause of openmp task construct are not visible inside lldb debugger.
This is because compiler does not generate debug info for it.

Please consider the testcase debug_private.c attached with patch.

```
   28   #pragma omp task shared(res) private(priv1, priv2) firstprivate(fpriv)
   29       {
   30         priv1 = n;
   31         priv2 = n + 2;
   32         printf("Task n=%d,priv1=%d,priv2=%d,fpriv=%d\n",n,priv1,priv2,fpriv);
   33
-> 34         res = priv1 + priv2 + fpriv + foo(n - 1);
   35       }
   36   #pragma omp taskwait
   37       return res;
(lldb) p priv1
error: <user expression 0>:1:1: use of undeclared identifier 'priv1'
priv1
^
(lldb) p priv2
error: <user expression 1>:1:1: use of undeclared identifier 'priv2'
priv2
^
(lldb) p fpriv
error: <user expression 2>:1:1: use of undeclared identifier 'fpriv'
fpriv
^
```

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

```
(lldb) p priv1
(int) $0 = 10
(lldb) p priv2
(int) $1 = 12
(lldb) p fpriv
(int) $2 = 14
```

Reviewed By: djtodoro

Differential Revision: https://reviews.llvm.org/D114504