Make sure to insert `Sized` bound first into clauses list
#120323 made it so that we don't insert an implicit `Sized` bound whenever we see an *explicit* `Sized` bound. However, since the code that inserts implicit sized bounds puts the bound as the *first* in the list, that means that it had the **side-effect** of possibly meaning we check `Sized` *after* checking other trait bounds.
If those trait bounds result in ambiguity or overflow or something, it may change how we winnow candidates. (**edit: SEE** #123303) This is likely the cause for the regression in https://github.com/rust-lang/rust/issues/123279#issuecomment-2028899598, since the impl...
```rust
impl<T: Job + Sized> AsJob for T { // <----- changing this to `Sized + Job` or just `Job` (which turns into `Sized + Job`) will FIX the issue.
}
```
...looks incredibly suspicious.
Fixes [after beta-backport] #123279.
Alternative is to revert #120323. I don't have a strong opinion about this, but think it may be nice to keep the diagnostic changes around.