Remove DiagnosticBuilderState.

Currently it's used for two dynamic checks:
- When a diagnostic is emitted, has it been emitted before?
- When a diagnostic is dropped, has it been emitted/cancelled?

The first check is no longer need, because `emit` is consuming, so it's
impossible to emit a `DiagnosticBuilder` twice. The second check is
still needed.

This commit replaces `DiagnosticBuilderState` with a simpler
`Option<Box<Diagnostic>>`, which is enough for the second check:
functions like `emit` and `cancel` can take the `Diagnostic` and then
`drop` can check that the `Diagnostic` was taken.

The `DiagCtxt` reference from `DiagnosticBuilderState` is now stored as
its own field, removing the need for the `dcx` method.

As well as making the code shorter and simpler, the commit removes:
- One (deprecated) `ErrorGuaranteed::unchecked_claim_error_was_emitted`
  call.
- Two `FIXME(eddyb)` comments that are no longer relevant.
- The use of a dummy `Diagnostic` in `into_diagnostic`.

Nice!

compiler/rustc_mir_transform/src/errors.rs

@@ -181,8 +181,7 @@ pub(crate) struct UnsafeOpInUnsafeFn {
 impl<'a> DecorateLint<'a, ()> for UnsafeOpInUnsafeFn {
     #[track_caller]
     fn decorate_lint<'b>(self, diag: &'b mut DiagnosticBuilder<'a, ()>) {
-        let dcx = diag.dcx().expect("lint should not yet be emitted");
-        let desc = dcx.eagerly_translate_to_string(self.details.label(), [].into_iter());
+        let desc = diag.dcx.eagerly_translate_to_string(self.details.label(), [].into_iter());
         diag.arg("details", desc);
         diag.span_label(self.details.span, self.details.label());
         self.details.add_subdiagnostics(diag);