[clang] [clang] Improve Ofast Warning (PR #183002)

[Sam Elliott via cfe-commits]

lenary wrote:

Yeah, indeed. We had reached the conclusion that the behaviour is complex if there are two optimisation levels specified and one is `-Ofast`. IMO this means if you have `-Ofast` and another level, you need the warning, in addition to if just `-Ofast` is specified. The easiest way to detect this is to just look for `-Ofast`.

I do want us to support users that have build systems where e.g. `-Ofast` is in their default flags, and then they add flags to override the effective optimisation level for 

I meant to confirm that there is still a `-Wno-error=` option that ensures `-Werror` users can stop this breaking their build - and indeed `-Wno-error=deprecated-ofast` works for this.

https://github.com/llvm/llvm-project/pull/183002