check_match: fix handling of privately uninhabited types

the match-checking code used to use TyErr for signaling "unknown,
inhabited" types for a long time. It had been switched to using the
exact type in #38069, to handle uninhabited types.

However, in #39980, we discovered that we still needed the "unknown
inhabited" logic, but I used `()` instead of `TyErr` to handle that.
Revert to using `TyErr` to fix that problem.

src/test/run-pass/issue-46964.rs

@@ -0,0 +1,28 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+mod my_mod {
+    #[derive(Clone, Copy, Eq, PartialEq, PartialOrd, Ord, Hash)]
+    pub struct Name<'a> {
+        source: &'a str,
+    }
+
+    pub const JSON: Name = Name { source: "JSON" };
+}
+
+pub fn crash() -> bool {
+  match (my_mod::JSON, None) {
+    (_, Some(my_mod::JSON)) => true,
+    (my_mod::JSON, None) => true,
+    _ => false,
+  }
+}
+
+fn main() {}