Tweak auto trait errors

Make suggestions to remove params and super traits tool-only, and make
the suggestion span more accurate.

```
error[E0567]: auto traits cannot have generic parameters
  --> $DIR/auto-trait-validation.rs:6:19
   |
LL | auto trait Generic<T> {}
   |            -------^^^
   |            |
   |            auto trait cannot have generic parameters

error[E0568]: auto traits cannot have super traits or lifetime bounds
  --> $DIR/auto-trait-validation.rs:8:20
   |
LL | auto trait Bound : Copy {}
   |            -----   ^^^^
   |            |
   |            auto traits cannot have super traits or lifetime bounds
```

```
error[E0380]: auto traits cannot have associated items
  --> $DIR/issue-23080.rs:5:8
   |
LL | unsafe auto trait Trait {
   |                   ----- auto traits cannot have associated items
LL |     fn method(&self) {
   |        ^^^^^^
```

1 files changed, 10 insertions, 6 deletions

diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index 895a457ec1d..e99e5cc4ae6 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -699,19 +699,23 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn deny_super_traits(&self, bounds: &GenericBounds, ident_span: Span) {
+    fn deny_super_traits(&self, bounds: &GenericBounds, ident: Span) {
         if let [.., last] = &bounds[..] {
-            let span = ident_span.shrink_to_hi().to(last.span());
-            self.dcx().emit_err(errors::AutoTraitBounds { span, ident: ident_span });
+            let span = bounds.iter().map(|b| b.span()).collect();
+            let removal = ident.shrink_to_hi().to(last.span());
+            self.dcx().emit_err(errors::AutoTraitBounds { span, removal, ident });
         }
     }
 
-    fn deny_where_clause(&self, where_clause: &WhereClause, ident_span: Span) {
+    fn deny_where_clause(&self, where_clause: &WhereClause, ident: Span) {
         if !where_clause.predicates.is_empty() {
             // FIXME: The current diagnostic is misleading since it only talks about
             // super trait and lifetime bounds while we should just say “bounds”.
-            self.dcx()
-                .emit_err(errors::AutoTraitBounds { span: where_clause.span, ident: ident_span });
+            self.dcx().emit_err(errors::AutoTraitBounds {
+                span: vec![where_clause.span],
+                removal: where_clause.span,
+                ident,
+            });
         }
     }