[llvm] [Frontend][OpenMP] Reduction modifier must be applied somewhere (PR #92160)

[Tom Eccles via llvm-commits]

https://github.com/tblah commented:

If it isn't too time consuming, please could you add a test for this to the flang test suite so that we catch this bug with the regular build bots next time.

https://github.com/llvm/llvm-project/pull/92160