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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> flang -E is already consistent here, it always produces fixed form output. 

It would be good to document and to test that - perhaps that's already the case?

> So I think we can just teach flang -fc1 that '.i'

Agreed, and this should be trivial to implement. Thanks @DavidTruby !

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