Fix autofix for self and self as … in unused_imports lint

This fixes two problems with the autofixes for the `unused_imports`
lint:

- `use std::collections::{HashMap, self as coll};` would suggest, when
  `HashMap` is unused, the incorrect `use std::collections::self as coll;`
  which does not compile.

- `use std::borrow::{self, Cow};` would suggest, when `self` is unused,
  `use std::borrow::{Cow};`, which contains unnecessary brackets.

compiler/rustc_resolve/src/check_unused.rs

@@ -337,7 +337,8 @@ fn calc_unused_spans(
                     }
                 }
                 contains_self |= use_tree.prefix == kw::SelfLower
-                    && matches!(use_tree.kind, ast::UseTreeKind::Simple(None));
+                    && matches!(use_tree.kind, ast::UseTreeKind::Simple(_))
+                    && !unused_import.unused.contains(&use_tree_id);
                 previous_unused = remove.is_some();
             }
             if unused_spans.is_empty() {