[clang] [flang] [Flang] Add -ffast-real-mod and direct code for MOD on REAL types (PR #160660)

[Tom Eccles via cfe-commits]

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

Thanks for the updates. LGTM once Slava and Tarun are happy

https://github.com/llvm/llvm-project/pull/160660