Hang in the test-case from #1127 with extra assertions.

With the fix for the panic in #1128, we get through it and generate code correctly, but on llvm 4.0 we generate a cyclic alias reference, so we get stuck resolving items in assert_every_item_in_a_module (enabled only with extra assertions).

The test is:

// bindgen-flags: -- -std=c++11

template <typename> struct A;
template <typename, typename, bool> struct _Map_base;
template <typename _Pair, typename _Traits>
struct _Map_base<_Pair, _Traits, true> {
  using __hashtable_base = A<_Traits>;
  using key_type = typename __hashtable_base::key_type;
  using mapped_type = typename _Pair::type;
  mapped_type &at(const key_type &);
};
template <typename _Pair, typename _Traits>
auto _Map_base<_Pair, _Traits, true>::at(const key_type &) -> mapped_type & {}

The method definition does surprisingly matter, so there's something fishy going on there, but I'm punting on landing the test-case for now because the fix fixes the issue, and also unblocks updating bindgen in mozilla-central.

Start with the provided C++11 reproducer and the extra-assertions path in assert_every_item_in_a_module, comparing behavior on LLVM 4.0. Trace why the generated cyclic alias is not resolved and confirm that the hang no longer occurs after the issue is addressed; the report notes that the method definition is significant.