fix: don't replace default members' body
cc #12779, #12821
addresses https://github.com/rust-lang/rust-analyzer/pull/12821#issuecomment-1190157506
`gen_trait_fn_body()` only attempts to implement required trait member functions, so we shouldn't call it for `Implement default members` assist.
This patch also documents the precondition of `gen_trait_fn_body()` and inserts `debug_assert!`, but I'm not entirely sure if the assertions are appropriate.