[flang] Do not re-localize loop ivs when nested inside `block`s (#153350)

Consider the following example:
```fortran
  implicit none
  integer :: i, j

  do concurrent (i=1:10) local(j)
    block
      do j=1,20
      end do
    end block
  end do
```

Without the fix introduced in this PR, the compiler would "re-localize"
the `j` variable inside the `fir.do_concurrent` loop:
```mlir
    fir.do_concurrent {
      %7 = fir.alloca i32 {bindc_name = "j"}
      %8:2 = hlfir.declare %7 {uniq_name = "_QFloop_in_nested_blockEj"} : (!fir.ref<i32>) -> (!fir.ref<i32>, !fir.ref<i32>)
      ...
      fir.do_concurrent.loop (%arg0) = (%5) to (%6) step (%c1) local(@_QFloop_in_nested_blockEj_private_i32 %4#0 -> %arg1 : !fir.ref<i32>) {
        %12:2 = hlfir.declare %arg1 {uniq_name = "_QFloop_in_nested_blockEj"} : (!fir.ref<i32>) -> (!fir.ref<i32>, !fir.ref<i32>)
        ...
        %17:2 = fir.do_loop %arg2 = %14 to %15 step %c1_1 iter_args(%arg3 = %16) -> (index, i32) {
          fir.store %arg3 to %8#0 : !fir.ref<i32>
          ...
        }
      }
    }
```

This happened because we did a shallow look-up of `j` and since the loop
is nested inside a `block`, the look-up failed and we re-created a local
allocation for `j` inside the parent `fir.do_concurrent` loop. This
means that we ended up not using the actual localized symbol which is
passed as a region argument to the `fir.do_concurrent.loop` op.

In case of `j`, we do not need to do a shallow look-up. The shallow
look-up is only needed if a symbol is an OpenMP private one or an
iteration variable of a `do concurrent` loop. Neither of which applies
to `j`.

With the fix, `j` is properly resolved to the `local` region argument:
```mlir
    fir.do_concurrent {
      ...
      fir.do_concurrent.loop (%arg0) = (%5) to (%6) step (%c1) local(@_QFloop_in_nested_blockEj_private_i32 %4#0 -> %arg1 : !fir.ref<i32>) {
        ...
        %10:2 = hlfir.declare %arg1 {uniq_name = "_QFloop_in_nested_blockEj"} : (!fir.ref<i32>) -> (!fir.ref<i32>, !fir.ref<i32>)
        ...
        %15:2 = fir.do_loop %arg2 = %12 to %13 step %c1_1 iter_args(%arg3 = %14) -> (index, i32) {
          fir.store %arg3 to %10#0 : !fir.ref<i32>
          ...
        }
      }
    }
```