Add a test for mismatched types when comparing functions

src/test/compile-fail/fn-compare-mismatch.rs

@@ -0,0 +1,8 @@
+// xfail-stage0
+// error-pattern:expected fn() but found fn(int)
+
+fn main() {
+  fn f() {}
+  fn g(int i) {}
+  auto x = f == g;
+}