[clang] [flang] [Flang] Add -ffast-real-mod back for further control of MOD optimizations (PR #167118)

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu approved this pull request.

Thanks for all the changes. LGTM.

https://github.com/llvm/llvm-project/pull/167118