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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > The warning message is now emitted any time there is -Ofast in the command-line string, rather than only when -Ofast is the effective optimization level.
> 
> I'm not sure I see what the point in doing this is?

The flag is deprecated and someday (in the far future) might be removed at which point it will start to give a hard error when passed on the command line unless we go to special lengths to accept it and ignore it (https://godbolt.org/z/xn48sq85W), so it's better to always tell the user about the deprecation instead of only when the flag is the effective optimization level.

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