[clang] [clang] Add deprecation warning for `-Ofast` driver option (PR #98736)

[Kiran Chandramohan via cfe-commits]

kiranchandramohan wrote:

> > Could you add a flang driver test as well?
> 
> The RFC was specifically about `-Ofast` in Clang and not behavior in Flang, so what test behavior would you like to see? This patch currently deprecates for both Clang and Flang, is that acceptable?

I was asking for a test that checks for the warning in Flang. But if @Endilll cannot build flang, you can proceed with the patch. I will create a patch separately for Flang.

https://github.com/llvm/llvm-project/pull/98736