[Flang][OpenMP] Restructure recursive lowering in `createBodyOfOp` (#77761)

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

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Recursive lowering [5/5]

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Co-authored-by: Kiran Chandramohan <kiran.chandramohan@arm.com>