Auto merge of #119922 - nnethercote:fix-Diag-code-is_lint, r=oli-obk

Rework how diagnostic lints are stored.

`Diagnostic::code` has the type `DiagnosticId`, which has `Error` and
`Lint` variants. Plus `Diagnostic::is_lint` is a bool, which should be
redundant w.r.t. `Diagnostic::code`.

Seems simple. Except it's possible for a lint to have an error code, in
which case its `code` field is recorded as `Error`, and `is_lint` is
required to indicate that it's a lint. This is what happens with
`derive(LintDiagnostic)` lints. Which means those lints don't have a
lint name or a `has_future_breakage` field because those are stored in
the `DiagnosticId::Lint`.

It's all a bit messy and confused and seems unintentional.

This commit:
- removes `DiagnosticId`;
- changes `Diagnostic::code` to `Option<String>`, which means both
  errors and lints can straightforwardly have an error code;
- changes `Diagnostic::is_lint` to `Option<IsLint>`, where `IsLint` is a
  new type containing a lint name and a `has_future_breakage` bool, so
  all lints can have those, error code or not.

r? `@oli-obk`

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -41,8 +41,8 @@ pub enum TypeAnnotationNeeded {
     E0284,
 }
 
-impl Into<rustc_errors::DiagnosticId> for TypeAnnotationNeeded {
-    fn into(self) -> rustc_errors::DiagnosticId {
+impl Into<String> for TypeAnnotationNeeded {
+    fn into(self) -> String {
         match self {
             Self::E0282 => rustc_errors::error_code!(E0282),
             Self::E0283 => rustc_errors::error_code!(E0283),