[clang] [flang] [Flang][Driver] Deprecate Ofast (PR #101701)

[Kiran Chandramohan via cfe-commits]

kiranchandramohan wrote:

I did not see any opposition in https://discourse.llvm.org/t/rfc-deprecate-ofast-in-flang/80243. So would like to land this before llvm 20.

https://github.com/llvm/llvm-project/pull/101701