[clang] [flang] [flang] Preserve fixed form in fc1 -x value (PR #117563)

[via cfe-commits]

macurtis-amd wrote:

(Sorry for the delayed response ... was on Thanksgiving holiday)

@banach-space @DavidTruby Thanks for looking at this!

Latest version of the fix simply adds `-ffixed-form` for pre-processed input, unless the user manually specifies `-ffixed-form` or `-ffree-form`.

https://github.com/llvm/llvm-project/pull/117563