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`.

Author: nnethercote

src/diagnostics.rs

@@ -455,7 +455,7 @@ pub fn report_msg<'tcx>(
     let sess = machine.tcx.sess;
     let level = match diag_level {
         DiagLevel::Error => Level::Error,
-        DiagLevel::Warning => Level::Warning(None),
+        DiagLevel::Warning => Level::Warning,
         DiagLevel::Note => Level::Note,
     };
     let mut err = DiagnosticBuilder::<()>::new(sess.dcx(), level, title);