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

[Michael Klemm via cfe-commits]

mjklemm wrote:

> Thanks for the changes.
> 
> The tests in `Lower/Intrinsics/fast-real-mod.f90` now test that the options are correctly handled by `-fc1` both with and without `-ffast-math` and when both `-ffast-real-mod` and `-ffast-no-real-mod` are provided. That should be checked in the driver as well. Arguably, it is better to check this in the driver since users are not expected to use `-fc1` directly, but they may provide both options to the driver.

I have updated that test.

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