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

[Sam Elliott via cfe-commits]

lenary wrote:

Ok, here's where I've got to (with help from @efriedma-quic )

- I've pushed most of the complexity into the docs
- The warning still guides people towards `-O3` (or pushes them to the docs)
- The warning is present if `-Ofast` is present, not just if it is the effective optimisation level.
- The docs now try to accurately summarise how `-Ofast` actually works
- There are now docs links between the Optimisation Levels and the strict aliasing docs, if I got the syntax right.



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