[flang][fir] Add MLIR op for `do concurrent` (#130893)

Adds new MLIR ops to model `do concurrent`. In order to make `do
concurrent` representation self-contained, a loop is modeled using 2
ops, one wrapper and one that contains the actual body of the loop. For
example, a 2D `do concurrent` loop is modeled as follows:

```mlir
  fir.do_concurrent {
    %i = fir.alloca i32
    %j = fir.alloca i32
    fir.do_concurrent.loop
      (%i_iv, %j_iv) = (%i_lb, %j_lb) to (%i_ub, %j_ub) step (%i_st, %j_st) {
      %0 = fir.convert %i_iv : (index) -> i32
      fir.store %0 to %i : !fir.ref<i32>

      %1 = fir.convert %j_iv : (index) -> i32
      fir.store %1 to %j : !fir.ref<i32>
    }
  }
```

The `fir.do_concurrent` wrapper op encapsulates both the actual loop and
the allocations required for the iteration variables. The
`fir.do_concurrent.loop` op is a multi-dimensional op that contains the
loop control and body. See the ops' docs for more info.