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_builtin_macros/src/test.rs

@@ -394,7 +394,7 @@ fn not_testable_error(cx: &ExtCtxt<'_>, attr_sp: Span, item: Option<&ast::Item>)
     let level = match item.map(|i| &i.kind) {
         // These were a warning before #92959 and need to continue being that to avoid breaking
         // stable user code (#94508).
-        Some(ast::ItemKind::MacCall(_)) => Level::Warning(None),
+        Some(ast::ItemKind::MacCall(_)) => Level::Warning,
         _ => Level::Error,
     };
     let mut err = DiagnosticBuilder::<()>::new(dcx, level, msg);