Rollup merge of #154449 - nnethercote:rustc_errors-dep-rustc_abi, r=davidtwco

Invert dependency between `rustc_errors` and `rustc_abi`.

Currently, `rustc_errors` depends on `rustc_abi`, which depends on `rustc_error_messages`. This is a bit odd.

`rustc_errors` depends on `rustc_abi` for a single reason: `rustc_abi` defines a type `TargetDataLayoutErrors` and `rustc_errors` impls `Diagnostic` for that type.

We can get a more natural relationship by inverting the dependency, moving the `Diagnostic` trait upstream. Then `rustc_abi` defines `TargetDataLayoutErrors` and also impls `Diagnostic` for it. `rustc_errors` is already pretty far upstream in the crate graph, it doesn't hurt to push it a little further because errors are a very low-level concept.

r? @davidtwco