red-knot: infer and display ellipsis type (#13124)

## Summary

Just what it says on the tin: adds basic `EllipsisType` inference for
any time `...` appears in the AST.

## Test Plan

Test that `x = ...` produces exactly what we would expect.

---------

Co-authored-by: Carl Meyer <carl@oddbird.net>

crates/red_knot_python_semantic/src/types/infer.rs

@@ -1337,8 +1337,7 @@ impl<'db> TypeInferenceBuilder<'db> {
         &mut self,
         _literal: &ast::ExprEllipsisLiteral,
     ) -> Type<'db> {
-        // TODO Ellipsis
-        Type::Unknown
+        builtins_symbol_ty_by_name(self.db, "Ellipsis")
     }
 
     fn infer_tuple_expression(&mut self, tuple: &ast::ExprTuple) -> Type<'db> {
@@ -2470,6 +2469,24 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn ellipsis_type() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "src/a.py",
+            "
+            x = ...
+            ",
+        )?;
+
+        // TODO: update this once `infer_ellipsis_literal_expression` correctly
+        // infers `types.EllipsisType`.
+        assert_public_ty(&db, "src/a.py", "x", "Unknown | Literal[EllipsisType]");
+
+        Ok(())
+    }
+
     #[test]
     fn resolve_union() -> anyhow::Result<()> {
         let mut db = setup_db();