Rollup merge of #117505 - estebank:issue-117501, r=TaKO8Ki

Fix incorrect trait bound restriction suggestion

Suggest

```
error[E0308]: mismatched types
  --> $DIR/restrict-assoc-type-of-generic-bound.rs:9:12
   |
LL | pub fn foo<A: MyTrait, B>(a: A) -> B {
   |                        -           - expected `B` because of return type
   |                        |
   |                        expected this type parameter
LL |     return a.bar();
   |            ^^^^^^^ expected type parameter `B`, found associated type
   |
   = note: expected type parameter `B`
             found associated type `<A as MyTrait>::T`
help: consider further restricting this bound
   |
LL | pub fn foo<A: MyTrait<T = B>, B>(a: A) -> B {
   |                      +++++++
```

instead of

```
error[E0308]: mismatched types
  --> $DIR/restrict-assoc-type-of-generic-bound.rs:9:12
   |
LL | pub fn foo<A: MyTrait, B>(a: A) -> B {
   |                        -           - expected `B` because of return type
   |                        |
   |                        expected this type parameter
LL |     return a.bar();
   |            ^^^^^^^ expected type parameter `B`, found associated type
   |
   = note: expected type parameter `B`
             found associated type `<A as MyTrait>::T`
help: consider further restricting this bound
   |
LL | pub fn foo<A: MyTrait + <T = B>, B>(a: A) -> B {
   |                      +++++++++
```

Fix #117501.

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -274,6 +274,8 @@ pub fn suggest_constraining_type_params<'a>(
                 span,
                 if span_to_replace.is_some() {
                     constraint.clone()
+                } else if constraint.starts_with("<") {
+                    constraint.to_string()
                 } else if bound_list_non_empty {
                     format!(" + {constraint}")
                 } else {