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.

compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs

@@ -9,8 +9,8 @@ use crate::emitter::FileWithAnnotatedLines;
 use crate::snippet::Line;
 use crate::translation::{to_fluent_args, Translate};
 use crate::{
-    CodeSuggestion, Diagnostic, DiagnosticId, DiagnosticMessage, Emitter, FluentBundle,
-    LazyFallbackBundle, Level, MultiSpan, Style, SubDiagnostic,
+    CodeSuggestion, Diagnostic, DiagnosticMessage, Emitter, FluentBundle, LazyFallbackBundle,
+    Level, MultiSpan, Style, SubDiagnostic,
 };
 use annotate_snippets::{Annotation, AnnotationType, Renderer, Slice, Snippet, SourceAnnotation};
 use rustc_data_structures::sync::Lrc;
@@ -127,7 +127,7 @@ impl AnnotateSnippetEmitter {
         level: &Level,
         messages: &[(DiagnosticMessage, Style)],
         args: &FluentArgs<'_>,
-        code: &Option<DiagnosticId>,
+        code: &Option<String>,
         msp: &MultiSpan,
         _children: &[SubDiagnostic],
         _suggestions: &[CodeSuggestion],
@@ -181,11 +181,7 @@ impl AnnotateSnippetEmitter {
             let snippet = Snippet {
                 title: Some(Annotation {
                     label: Some(&message),
-                    id: code.as_ref().map(|c| match c {
-                        DiagnosticId::Error(val) | DiagnosticId::Lint { name: val, .. } => {
-                            val.as_str()
-                        }
-                    }),
+                    id: code.as_deref(),
                     annotation_type: annotation_type_for_level(*level),
                 }),
                 footer: vec![],