Add xfailed test for #3907

Author: brson

src/test/auxiliary/trait_typedef_cc.rs

@@ -0,0 +1,3 @@
+pub trait Foo {
+    fn bar();
+}

src/test/run-pass/trait-typedef-cc.rs

@@ -0,0 +1,20 @@
+// xfail-test FIXME: #3907
+// aux-build:trait_typedef_cc.rs
+extern mod trait_typedef_cc;
+
+type Foo = trait_typedef_cc::Foo;
+
+struct S {
+    name: int
+}
+
+impl S: Foo {
+    fn bar() { }
+}
+
+fn main() {
+    let s = S {
+        name: 0
+    };
+    s.bar();
+}