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/diagnostic.rs

@@ -152,7 +152,6 @@ pub enum DiagnosticId {
         name: String,
         /// Indicates whether this lint should show up in cargo's future breakage report.
         has_future_breakage: bool,
-        is_force_warn: bool,
     },
 }
 
@@ -248,7 +247,8 @@ impl Diagnostic {
                 true
             }
 
-            Level::Warning(_)
+            Level::ForceWarning(_)
+            | Level::Warning
             | Level::Note
             | Level::OnceNote
             | Level::Help
@@ -262,7 +262,7 @@ impl Diagnostic {
         &mut self,
         unstable_to_stable: &FxIndexMap<LintExpectationId, LintExpectationId>,
     ) {
-        if let Level::Expect(expectation_id) | Level::Warning(Some(expectation_id)) =
+        if let Level::Expect(expectation_id) | Level::ForceWarning(Some(expectation_id)) =
             &mut self.level
         {
             if expectation_id.is_stable() {
@@ -292,8 +292,11 @@ impl Diagnostic {
     }
 
     pub(crate) fn is_force_warn(&self) -> bool {
-        match self.code {
-            Some(DiagnosticId::Lint { is_force_warn, .. }) => is_force_warn,
+        match self.level {
+            Level::ForceWarning(_) => {
+                assert!(self.is_lint);
+                true
+            }
             _ => false,
         }
     }
@@ -472,7 +475,7 @@ impl Diagnostic {
     /// Add a warning attached to this diagnostic.
     #[rustc_lint_diagnostics]
     pub fn warn(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self {
-        self.sub(Level::Warning(None), msg, MultiSpan::new());
+        self.sub(Level::Warning, msg, MultiSpan::new());
         self
     }
 
@@ -484,7 +487,7 @@ impl Diagnostic {
         sp: S,
         msg: impl Into<SubdiagnosticMessage>,
     ) -> &mut Self {
-        self.sub(Level::Warning(None), msg, sp.into());
+        self.sub(Level::Warning, msg, sp.into());
         self
     }