Add a test case for empty mutable vector literals, XFAIL'd in rustboot

src/Makefile

@@ -456,6 +456,7 @@ TEST_XFAILS_BOOT :=  $(TASK_XFAILS) \
                     test/run-pass/obj-as.rs \
                     test/run-pass/vec-slice.rs \
                     test/run-pass/fn-lval.rs \
+                    test/run-pass/empty-mutable-vec.rs \
                     test/run-pass/generic-bind-2.rs \
                     test/run-pass/generic-fn-box.rs \
                     test/run-pass/generic-tup.rs \

src/test/run-pass/empty-mutable-vec.rs

@@ -0,0 +1,4 @@
+fn main() {
+    let vec[mutable int] v = vec(mutable);
+}
+