compiler: Treat ForceWarning as a Warning for diagnostic level

This silences an ICE.

compiler/rustc_errors/src/lib.rs

@@ -1529,7 +1529,7 @@ impl DiagCtxtInner {
             // Future breakages aren't emitted if they're `Level::Allow` or
             // `Level::Expect`, but they still need to be constructed and
             // stashed below, so they'll trigger the must_produce_diag check.
-            assert_matches!(diagnostic.level, Error | Warning | Allow | Expect);
+            assert_matches!(diagnostic.level, Error | ForceWarning | Warning | Allow | Expect);
             self.future_breakage_diagnostics.push(diagnostic.clone());
         }