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

[Slava Zakharin via cfe-commits]

vzakhari wrote:

Thank you, @mjklemm. It looks okay to me, modulo Tarun's comments.

If possible, can you please list the apps where you find new MOD handling preferrable or/and where using the fast MOD causes any accuracy issues. This will help others with triaging any testing failures.  I think it is worth creating a small doc in `flang/docs` about fast MOD options and where they may matter.

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