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

[Aaron Ballman via cfe-commits]

AaronBallman 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?

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