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_resolve/src/late.rs

@@ -16,7 +16,7 @@ use rustc_ast::ptr::P;
 use rustc_ast::visit::{self, AssocCtxt, BoundKind, FnCtxt, FnKind, Visitor};
 use rustc_ast::*;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
-use rustc_errors::{Applicability, DiagnosticArgValue, DiagnosticId, IntoDiagnosticArg};
+use rustc_errors::{Applicability, DiagnosticArgValue, IntoDiagnosticArg};
 use rustc_hir::def::Namespace::{self, *};
 use rustc_hir::def::{self, CtorKind, DefKind, LifetimeRes, NonMacroAttrKind, PartialRes, PerNS};
 use rustc_hir::def_id::{DefId, LocalDefId, CRATE_DEF_ID, LOCAL_CRATE};
@@ -531,7 +531,7 @@ impl<'a> PathSource<'a> {
         }
     }
 
-    fn error_code(self, has_unexpected_resolution: bool) -> DiagnosticId {
+    fn error_code(self, has_unexpected_resolution: bool) -> String {
         use rustc_errors::error_code;
         match (self, has_unexpected_resolution) {
             (PathSource::Trait(_), true) => error_code!(E0404),
@@ -3759,7 +3759,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             swap(&mut err.span, &mut parent_err.span);
             err.children = take(&mut parent_err.children);
             err.sort_span = parent_err.sort_span;
-            err.is_lint = parent_err.is_lint;
+            err.is_lint = parent_err.is_lint.clone();
 
             // merge the parent's suggestions with the typo suggestions
             fn append_result<T, E>(res1: &mut Result<Vec<T>, E>, res2: Result<Vec<T>, E>) {

compiler/rustc_resolve/src/lib.rs

@@ -257,7 +257,7 @@ enum ResolutionError<'a> {
         kind: &'static str,
         trait_path: String,
         trait_item_span: Span,
-        code: rustc_errors::DiagnosticId,
+        code: String,
     },
     /// Error E0201: multiple impl items for the same trait item.
     TraitImplDuplicate { name: Symbol, trait_item_span: Span, old_span: Span },