[MLIR][OpenMP] Add `private` clause to `omp.parallel` (#81452)

Extends the `omp.parallel` op by adding a `private` clause to model
[first]private variables. This uses the `omp.private` op to map
privatized variables to their corresponding privatizers.

Example `omp.private` op with `private` variable:
```
omp.parallel private(@x.privatizer %arg0 -> %arg1 : !llvm.ptr) {
  ^bb0(%arg1: !llvm.ptr):
    // ... use %arg1 ...
    omp.terminator
}
```

Whether the variable is private or firstprivate is determined by the
attributes of the corresponding `omp.private` op.