Change how force-warn lint diagnostics are recorded.

`is_force_warn` is only possible for diagnostics with `Level::Warning`,
but it is currently stored in `Diagnostic::code`, which every diagnostic
has.

This commit:
- removes the boolean `DiagnosticId::Lint::is_force_warn` field;
- adds a `ForceWarning` variant to `Level`.

Benefits:
- The common `Level::Warning` case now has no arguments, replacing
  lots of `Warning(None)` occurrences.
- `rustc_session::lint::Level` and `rustc_errors::Level` are more
  similar, both having `ForceWarning` and `Warning`.

compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs

@@ -87,7 +87,7 @@ fn source_string(file: Lrc<SourceFile>, line: &Line) -> String {
 fn annotation_type_for_level(level: Level) -> AnnotationType {
     match level {
         Level::Bug | Level::DelayedBug | Level::Fatal | Level::Error => AnnotationType::Error,
-        Level::Warning(_) => AnnotationType::Warning,
+        Level::ForceWarning(_) | Level::Warning => AnnotationType::Warning,
         Level::Note | Level::OnceNote => AnnotationType::Note,
         Level::Help | Level::OnceHelp => AnnotationType::Help,
         // FIXME(#59346): Not sure how to map this level