[OPENMP50]Codegen for scan directives in parallel for regions.

Summary:
Added codegen for scan directives in parallel for regions.

Emits the code for the directive with inscan reductions.
Original code:
```
 #pragma omp parallel for reduction(inscan, op : ...)
 for() {
   <input phase>;
   #pragma omp scan (in)exclusive(...)
   <scan phase>
 }
```
is transformed to something:

```
 #pragma omp parallel
{
size num_iters = <num_iters>;
<type> buffer[num_iters];
 #pragma omp for
for (i: 0..<num_iters>) {
  <input phase>;
  buffer[i] = red;
}
 #pragma omp barrier
for (int k = 0; k != ceil(log2(num_iters)); ++k)
for (size cnt = last_iter; cnt >= pow(2, k); --k)
  buffer[i] op= buffer[i-pow(2,k)];
 #pragma omp for
for (0..<num_iters>) {
  red = InclusiveScan ? buffer[i] : buffer[i-1];
  <scan phase>;
}
}
```

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, sstefan1, cfe-commits, caomhin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81478