This extracts a new `parse_cfg` function that's used between both.
- Treat `#[doc(cfg(x), cfg(y))]` the same as `#[doc(cfg(x)]
#[doc(cfg(y))]`. Previously it would be completely ignored.
- Treat `#[doc(inline, cfg(x))]` the same as `#[doc(inline)]
#[doc(cfg(x))]`. Previously, the cfg would be ignored.
- Pass the cfg predicate through to rustc_expand to be validated
Co-authored-by: Vadim Petrochenkov <vadim.petrochenkov@gmail.com>
