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.

Cargo.lock

@@ -3504,6 +3504,7 @@ dependencies = [
  "rand_xoshiro",
  "rustc_data_structures",
  "rustc_error_messages",
+ "rustc_errors",
  "rustc_hashes",
  "rustc_index",
  "rustc_macros",
@@ -3908,7 +3909,6 @@ dependencies = [
  "anstream",
  "anstyle",
  "derive_setters",
- "rustc_abi",
  "rustc_ast",
  "rustc_data_structures",
  "rustc_error_codes",