[clang] [Driver] Restrict Ofast deprecation help message to Clang (PR #101682)

[Valentin Clement バレンタイン クレメン via cfe-commits]

clementval wrote:

> Thanks for the clarification! Cc some flang developers to confirm this and #101701 are the direction as https://discourse.llvm.org/t/rfc-deprecate-ofast-in-flang/80243 doesn't have lots of recorded discussion: @banach-space @clementval @klausler

👍  This was discussed during the flang weekly call. 

https://github.com/llvm/llvm-project/pull/101682