Rollup merge of #76695 - iximeow:trait-generic-bound-suggestion, r=estebank

fix syntax error in suggesting generic constraint in trait parameter

suggest `where T: Foo` for the first bound on a trait, then suggest
`, T: Foo` when the suggested bound would add to an existing set of
`where` clauses. `where T: Foo` may be the first bound if `T` has a
default, because we'd rather suggest
```
trait A<T=()> where T: Copy
```
than
```
trait A<T: Copy=()>
```
for legibility reasons.

the test case i added here is derived from [this reproduction](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=0bf3ace9f2a183d0bdbd748c6b8e3971):
```
struct B<T: Copy> {
    t: T
}

trait A<T = ()> {
    fn returns_constrained_type(&self, t: T) -> B<T> {
        B { t }
    }
}
```
where the suggested fix,
```
trait A<T = ()>, T: Copy { ... }
```
is in fact invalid syntax!

i also found an error in the existing suggestion for `trait Base<T = String>: Super<T>` where rustc would suggest `trait Base<T = String>: Super<T>, T: Copy`, but `T: Copy` is the first of the trait's `where` clauses and should be `where T: Copy` as well. the test for that suggestion expects invalid syntax, and has been revised to a compiler-pleasing `trait Base<T = String>: Super<T> where T: Copy`.

judging by https://github.com/rust-lang/rust/pull/70009 i'll.. cc @estebank ?

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