Diagnostic for `impl for S` should also suggest syntax for inherit impl

[jruderman]

Given the following code:

struct S { }
impl for S { }

The current output is:

error: missing trait in a trait impl
 --> src/lib.rs:2:5
  |
2 | impl for S { }
  |     ^

Ideally the output should look like:

help: to implement trait items for struct S, write `impl TraitName for S { ... }`
help: to implement inherit items for struct S, write `impl S { ... }`

Followup to #56031