Introduce allow-by-default lints for checking whether diagnostics are
written in `SessionDiagnostic`/`AddSubdiagnostic` impls and whether
diagnostics are translatable. These lints can be denied for modules once
they are fully migrated to impls and translation.

Signed-off-by: David Wood <david.wood@huawei.com>

…suggestion`

…nts, r=oli-obk

lint: add diagnostic translation migration lints

Introduce allow-by-default lints for checking whether diagnostics are written in
`SessionDiagnostic` or `AddSubdiagnostic` impls and whether diagnostics are translatable. These lints can be denied for modules once they are fully migrated to impls and translation.

These lints are intended to be temporary - once all diagnostics have been changed then we can just change the APIs we have and that will enforce these constraints thereafter.

r? ```@oli-obk```

interpret: unify offset_from check with offset check

`offset` does the check with a single `check_ptr_access` call while `offset_from` used two calls. Make them both just one one call.

I originally intended to actually factor this into a common function, but I am no longer sure if that makes a lot of sense... the two functions start with pretty different precondition (e.g. `offset` *knows* that the 2nd pointer has the same provenance).

I also reworded the UB messages a little. Saying it "cannot" do something is not how we usually phrase UB (as far as I know). Instead it's not *allowed* to do that.

r? ```@oli-obk```

…_update_is_probably_complete, r=oli-obk

Make `type_changing_struct_update` no longer an incomplete feature

After rust-lang#97705, I don't see what would make it incomplete anymore. `check_expr_struct_fields` seems to now implement the RFC to the letter.

r? ```@nikomatsakis```
cc ```@rust-lang/types```

…g, r=davidtwco

Remove unnecessary `to_string` and `String::new`

rust-lang@73fa217 changed the type of the `suggestion` argument to `impl ToString`. This patch removes unnecessary `to_string` and `String::new`.

cc: ```@davidtwco```