[flang] [llvm] [mlir] [flang][OpenMP] Lower target in_reduction for host fallback (PR #199967)

[Sergio Afonso via llvm-commits]

https://github.com/skatrak commented:

Thank you @Saieiei, only minor nits left from me. Otherwise, this looks almost ready.

https://github.com/llvm/llvm-project/pull/199967