[Flang][OpenMP] Restructure recursive lowering in `createBodyOfOp`

This brings `createBodyOfOp` to its final intended form. First, input
privatization is performed, then the recursive lowering takes place,
and finally the output privatization (lastprivate) is done.

This enables fixing a known issue with infinite loops inside of an
OpenMP region, and the fix is included in this patch.

Fixes https://github.com/llvm/llvm-project/issues/74348.

Recursive lowering [5/5]