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

[David Truby via cfe-commits]

DavidTruby wrote:

Thanks for the info on -save-temps, I understand it a little better now !

> we could simplify by requiring that flang -E always produces free-form output.

flang -E is already consistent here, it always produces fixed form output. So I think we can just teach flang -fc1 that '.i' files are fixed form. Or just pick another file extension to use ('.fpp'?) and teach it that that is fixed form? I suppose using '.i' doesn't _really_ matter.

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