Accept less invalid Rust in rustdoc
pulled out of https://github.com/rust-lang/rust/pull/117213 where this change was already approved
This only affects rustdoc, and has up to [20% perf regressions in rustdoc](https://github.com/rust-lang/rust/pull/117213#issuecomment-1785776288). These are unavoidable, as we are simply doing more checks now, but it's part of the longer term plan of making rustdoc more resistant to ICEs by only accepting valid Rust code.
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