[Flang][OpenMP] Accept the reduction modifier (#86492)

Accept the reduction modifier in the Flang parser. Issue a TODO message
during lowering.

OpenMP 5.0 introduced the reduction modifier. Details can be seen in
2.19.5.4 reductionClause.
OpenMP 5.2 relevant section is 5.5.8reductionClause.

This will help remove some of the parser errors highlighted in the
following post and also bring it to a well defined behaviour (produce
TODO errors for unsupported features, do not crash).
https://discourse.llvm.org/t/proposal-rename-flang-new-to-flang/69462/60