Auto merge of #10408 - Urgau:check-cfg-features, r=alexcrichton

Auto merge of #10408 - Urgau:check-cfg-features, r=alexcrichton

Add -Z check-cfg-features to enable compile-time checking of features

This pull-request implements the "[Cargo support](https://rust-lang.github.io/rfcs/3013-conditional-compilation-checking.html#cargo-support)" section of [RFC 3013: Checking conditional compilation at compile time](https://rust-lang.github.io/rfcs/3013-conditional-compilation-checking.html#checking-conditional-compilation-at-compile-time).

The support is added in the form of an new unstable flags: `-Z check-cfg-features` that pass all possible features of a package to
`rustc` unstable `--check-cfg` command line as `--check-cfg=values(feature, ...)`. This enables compile time checking of `feature` values in `#[cfg]`, `cfg!` and `#[cfg_attr]`.

This new flag currently only affects `rustc` but `rustdoc` support will be added as soon as [it's support](https://github.com/rust-lang/rust/pull/94154) is merged.

Note than the intent is that this command line options become the default when stabilizing as suggested in the RFC:
> [..] it seems uncontroversial for Cargo to enable checking for feature = "..." values immediately [..]