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_codegen_ssa/src/back/write.rs

@@ -14,7 +14,7 @@ use rustc_data_structures::memmap::Mmap;
 use rustc_data_structures::profiling::{SelfProfilerRef, VerboseTimingGuard};
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::emitter::Emitter;
-use rustc_errors::{translation::Translate, DiagCtxt, DiagnosticId, FatalError, Level};
+use rustc_errors::{translation::Translate, DiagCtxt, FatalError, Level};
 use rustc_errors::{DiagnosticBuilder, DiagnosticMessage, Style};
 use rustc_fs_util::link_or_copy;
 use rustc_hir::def_id::{CrateNum, LOCAL_CRATE};
@@ -1000,7 +1000,7 @@ type DiagnosticArgName<'source> = Cow<'source, str>;
 struct Diagnostic {
     msgs: Vec<(DiagnosticMessage, Style)>,
     args: FxHashMap<DiagnosticArgName<'static>, rustc_errors::DiagnosticArgValue<'static>>,
-    code: Option<DiagnosticId>,
+    code: Option<String>,
     lvl: Level,
 }