[clang] [flang] [flang] Complete alignment of -x language modes with gfortran (PR #133775)

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu commented:

Other than the nit, this looks ok. However, I must admit that I don't fully understand the nuances of the various modes, and I got a bit lost with the previous PR as well, so I'll leave it to someone else to approve as they see fit.

https://github.com/llvm/llvm-project/pull/133775