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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> We are deprecating -Ofast and I would prefer to hint users first at conforming behaviour -O3 and then as last resort at -O3 with -ffast-math.

@tschuett I'm sympathetic to this arguments, but I think it's no less important to tell users in no uncertain terms how to preserve their current behavior. If there's a wording that achieves both goals, I'd be happy to incorporate it into this PR.

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