[flang-commits] [flang] [flang][docs] Update the `-Ofast` description in `FlangDriver.md` (PR #180138)

[Kiran Chandramohan via flang-commits]

kiranchandramohan wrote:

Could you also update the relevant driver help option in clang/include/clang/Options/Options.td?

```
def Ofast : Joined<["-"], "Ofast">, Group<O_Group>,
  Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>,
  HelpTextForVariants<[FlangOption, FC1Option],
    "Deprecated; use '-O3 -ffast-math -fstack-arrays' for the same behavior,"
    " or '-O3 -fstack-arrays' to enable only conforming optimizations">,
  HelpText<"Deprecated; use '-O3 -ffast-math' for the same behavior,"
           " or '-O3' to enable only conforming optimizations">;
```

https://github.com/llvm/llvm-project/pull/180138