[clang] [flang] [flang][driver] Add pre-processing type to `.i` (pre-processed) files (PR #104664)

[Kareem Ergawy via cfe-commits]

ergawy wrote:

Thanks for taking a look! 🙏

> Is there any reference that would document .i files?

These are pre-processed Fortran source files. I do not know if these are standard or flang-new specific tbh. However, I updated the PR title and description with more info about these files and where they are emitted. Let me know if we can expand this further.

https://github.com/llvm/llvm-project/pull/104664