Overhaul how stashed diagnostics work, again.

Stashed errors used to be counted as errors, but could then be
cancelled, leading to `ErrorGuaranteed` soundness holes. #120828 changed
that, closing the soundness hole. But it introduced other difficulties
because you sometimes have to account for pending stashed errors when
making decisions about whether errors have occured/will occur and it's
easy to overlook these.

This commit aims for a middle ground.
- Stashed errors (not warnings) are counted immediately as emitted
  errors, avoiding the possibility of forgetting to consider them.
- The ability to cancel (or downgrade) stashed errors is eliminated, by
  disallowing the use of `steal_diagnostic` with errors, and introducing
  the more restrictive methods `try_steal_{modify,replace}_and_emit_err`
  that can be used instead.

Other things:
- `DiagnosticBuilder::stash` and `DiagCtxt::stash_diagnostic` now both
  return `Option<ErrorGuaranteed>`, which enables the removal of two
  `delayed_bug` calls and one `Ty::new_error_with_message` call. This is
  possible because we store error guarantees in
  `DiagCtxt::stashed_diagnostics`.
- Storing the guarantees also saves us having to maintain a counter.
- Calls to the `stashed_err_count` method are no longer necessary
  alongside calls to `has_errors`, which is a nice simplification, and
  eliminates two more `span_delayed_bug` calls and one FIXME comment.
- Tests are added for three of the four fixed PRs mentioned below.
- `issue-121108.rs`'s output improved slightly, omitting a non-useful
  error message.

Fixes #121451.
Fixes #121477.
Fixes #121504.
Fixes #121508.

compiler/rustc_parse/src/parser/expr.rs

@@ -1762,24 +1762,25 @@ impl<'a> Parser<'a> {
         err: impl FnOnce(&Self) -> Diag<'a>,
     ) -> L {
         assert!(could_be_unclosed_char_literal(ident));
-        if let Some(diag) = self.dcx().steal_diagnostic(ident.span, StashKey::LifetimeIsChar) {
-            diag.with_span_suggestion_verbose(
-                ident.span.shrink_to_hi(),
-                "add `'` to close the char literal",
-                "'",
-                Applicability::MaybeIncorrect,
-            )
-            .emit();
-        } else {
-            err(self)
-                .with_span_suggestion_verbose(
+        self.dcx()
+            .try_steal_modify_and_emit_err(ident.span, StashKey::LifetimeIsChar, |err| {
+                err.span_suggestion_verbose(
                     ident.span.shrink_to_hi(),
                     "add `'` to close the char literal",
                     "'",
                     Applicability::MaybeIncorrect,
-                )
-                .emit();
-        }
+                );
+            })
+            .unwrap_or_else(|| {
+                err(self)
+                    .with_span_suggestion_verbose(
+                        ident.span.shrink_to_hi(),
+                        "add `'` to close the char literal",
+                        "'",
+                        Applicability::MaybeIncorrect,
+                    )
+                    .emit()
+            });
         let name = ident.without_first_quote().name;
         mk_lit_char(name, ident.span)
     }