6043: Allow missing trait members assist without needing braces r=matklad a=M-J-Hooper

Assist to complete missing items when implementing a trait does not appear without impl def braces (see #5144 ).

The reason behind this was that this assist is based on `ast::AssocItemList` which only appears in the AST after the braces are added to the impl def.

Instead of relying on and replacing the item list, we now instead replace the entire `ast::Impl` and add the item list if its missing.

Co-authored-by: Matt Hooper <matthewjhooper94@gmail.com>