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_hir_analysis/src/structured_errors.rs

@@ -6,13 +6,13 @@ pub use self::{
     missing_cast_for_variadic_arg::*, sized_unsized_cast::*, wrong_number_of_generic_args::*,
 };
 
-use rustc_errors::{DiagnosticBuilder, DiagnosticId};
+use rustc_errors::DiagnosticBuilder;
 use rustc_session::Session;
 
 pub trait StructuredDiagnostic<'tcx> {
     fn session(&self) -> &Session;
 
-    fn code(&self) -> DiagnosticId;
+    fn code(&self) -> String;
 
     fn diagnostic(&self) -> DiagnosticBuilder<'tcx> {
         let err = self.diagnostic_common();