Don't warn about old rustdoc lint names (temporarily)
Since https://github.com/rust-lang/rust/pull/80527, rustdoc users have an unpleasant situation: they can either use the new tool lint names (`rustdoc::non_autolinks`) or they can use the old names (`non_autolinks`). If they use the tool lints, they get a hard error on stable compilers, because rustc rejects all tool names it doesn't recognize (https://github.com/rust-lang/rust/issues/66079#issuecomment-788589193). If they use the old name, they get a warning to rename the lint to the new name. The only way to compile without warnings is to add `#[allow(renamed_removed_lints)]`, which defeats the whole point of the change: we *want* people to switch to the new name.
To avoid people silencing the lint and never migrating to the tool lint, this avoids warning about the old name, while still allowing you to use the new name. Once the new `rustdoc` tool name makes it to the stable channel, we can change these lints to warn again.
This adds the new lint functions `register_alias` and `register_ignored` - I didn't see an existing way to do this.
r? `@Manishearth` cc `@rust-lang/rustdoc`
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