[clang] Fix erroneous warning. (PR #79821)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> Fix erroneous warning.

I'd suggest a more descriptive title, e.g. `[Driver] Fix erroneous warning for -fcx-limited-range and -fcx-fortran-rules`

Please also mention #70244 in the description. I am not familiar with the two options, I'll trust your judgement as the author of #70244 :)

https://github.com/llvm/llvm-project/pull/79821