Add test for non-exhaustive match failure.

src/Makefile

@@ -497,6 +497,7 @@ TEST_XFAILS_LLVM := $(TASK_XFAILS) \
                       explicit-fail.rs \
                       fail.rs \
                       linked-failure.rs \
+                      non-exhaustive-match.rs \
                       pred.rs \
                       str-overrun.rs \
                       vec-overrun.rs \

src/test/run-fail/non-exhaustive-match.rs

@@ -0,0 +1,15 @@
+// -*- rust -*-
+
+// error-pattern:non-exhaustive match failure
+
+tag t {
+  a;
+  b;
+}
+
+fn main() {
+  auto x = a;
+  alt (x) {
+    case (b) { }
+  }
+}