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

[Nico Weber via cfe-commits]

nico wrote:

…passing `Wno-msvc-not-found` as suggested above would also make the test go. But `-c` seems generally nicer here anyways?

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