[flang][OpenMP] Handle private/firstprivate clauses on sections construct

This patch adds private/firstprivate support for sections construct. For
a source like the following:

```
!$omp sections private(x) firstprivate(y)
    !$omp section
        <block of code>
    !$omp section
        <block of code>
!$omp end sections
```
...privatization proceeds to privatize `x` and `y` accordingly
inside each of the generated `omp.section` operations.

Reviewed By: peixin

Differential Revision: https://reviews.llvm.org/D131463