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

[Thorsten Schütt via cfe-commits]

tschuett 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.

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