[clang] [Clang][Basic] Enable `__has_feature(cfi)` (PR #151348)

[Florian Mayer via cfe-commits]

fmayer wrote:

> Also, I just learned that non-standard features are meant to be under `__has_extension` (`EXTENSION` macro) instead of `__has_feature`. See
> 
> https://github.com/llvm/llvm-project/blob/154354e60e46debb68716cde867cb4e325ce5eb7/clang/include/clang/Basic/Features.def#L20

Given `kcfi` already is in `has_feature`, maybe (if we decide to add this) it still is preferable to also put `cfi` there?

https://github.com/llvm/llvm-project/pull/151348