[flang][fir] Lower `do concurrent` loop nests to `fir.do_concurrent` (#132904)

Adds support for lowering `do concurrent` nests from PFT to the new
`fir.do_concurrent` MLIR op as well as its special terminator
`fir.do_concurrent.loop` which models the actual loop nest.

To that end, this PR emits the allocations for the iteration variables
within the block of the `fir.do_concurrent` op and creates a region for
the `fir.do_concurrent.loop` op that accepts arguments equal in number
to the number of the input `do concurrent` iteration ranges.

For example, given the following input:
```fortran
   do concurrent(i=1:10, j=11:20)
   end do
```
the changes in this PR emit the following MLIR:
```mlir
    fir.do_concurrent {
      %22 = fir.alloca i32 {bindc_name = "i"}
      %23:2 = hlfir.declare %22 {uniq_name = "_QFsub1Ei"} : (!fir.ref<i32>) -> (!fir.ref<i32>, !fir.ref<i32>)
      %24 = fir.alloca i32 {bindc_name = "j"}
      %25:2 = hlfir.declare %24 {uniq_name = "_QFsub1Ej"} : (!fir.ref<i32>) -> (!fir.ref<i32>, !fir.ref<i32>)
      fir.do_concurrent.loop (%arg1, %arg2) = (%18, %20) to (%19, %21) step (%c1, %c1_0) {
        %26 = fir.convert %arg1 : (index) -> i32
        fir.store %26 to %23#0 : !fir.ref<i32>
        %27 = fir.convert %arg2 : (index) -> i32
        fir.store %27 to %25#0 : !fir.ref<i32>
      }
    }
```

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