Add new minimal testcase showing generic tag memory corruption.

Author: graydon

src/test/run-pass/generic-tag-corruption.rs

@@ -0,0 +1,6 @@
+// xfail-stage0
+
+// This causes memory corruption in stage0.
+
+tag thing[K] { some(K); }
+fn main() { auto x = some("hi"); }