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_macros/src/diagnostics/diagnostic_builder.rs

@@ -211,7 +211,7 @@ impl DiagnosticDeriveVariantBuilder {
 
                     let code = nested.parse::<syn::LitStr>()?;
                     tokens.extend(quote! {
-                        diag.code(rustc_errors::DiagnosticId::Error(#code.to_string()));
+                        diag.code(#code.to_string());
                     });
                 } else {
                     span_err(path.span().unwrap(), "unknown argument")