rustc_trans: don't write discriminants for uninhabited variants

Fixes #46519.

Patch as suggested by eddyb:
https://github.com/rust-lang/rust/issues/46519#issuecomment-349443519

src/test/run-pass/issue-46519.rs

@@ -0,0 +1,37 @@
+// 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.
+
+// compile-flags:--test -O
+
+#[test]
+#[should_panic(expected = "creating inhabited type")]
+fn test() {
+    FontLanguageOverride::system_font(SystemFont::new());
+}
+
+pub enum FontLanguageOverride {
+    Normal,
+    Override(&'static str),
+    System(SystemFont)
+}
+
+pub enum SystemFont {}
+
+impl FontLanguageOverride {
+    fn system_font(f: SystemFont) -> Self {
+        FontLanguageOverride::System(f)
+    }
+}
+
+impl SystemFont {
+    fn new() -> Self {
+        panic!("creating inhabited type")
+    }
+}