Properly handle different defaults for severity of lints

Previously all lints were assumed to be `#[warn]`, and we had a hand-coded list of `#[allow]` exceptions. Now the severity is autogenerated from rustdoc output.

Also support lints that change status between editions, and the `warnings` lint group.

Author: ChayimFriedman2

crates/ide-completion/src/completions/attribute/lint.rs

@@ -11,7 +11,7 @@ pub(super) fn complete_lint(
     existing_lints: &[ast::Path],
     lints_completions: &[Lint],
 ) {
-    for &Lint { label, description } in lints_completions {
+    for &Lint { label, description, .. } in lints_completions {
         let (qual, name) = {
             // FIXME: change `Lint`'s label to not store a path in it but split the prefix off instead?
             let mut parts = label.split("::");