Unify rustc and rustdoc parsing of `cfg()`
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
Technically this is a breaking change, but doc_cfg is still nightly so I don't think it matters.
Fixes https://github.com/rust-lang/rust/issues/84437.
r? `````````@petrochenkov`````````
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