Add regression test

tests/cases/compiler/subtypeRelationForNever.ts

@@ -0,0 +1,6 @@
+function fail(message: string) : never { throw new Error(message); }
+function withFew<a, r>(values: a[], haveFew: (values: a[]) => r, haveNone: (reason: string) => r): r {
+    return values.length > 0 ? haveFew(values) : haveNone('No values.');
+}
+function id<a>(value: a) : a { return value; }
+const result = withFew([1, 2, 3], id, fail); // expected result is number[]