Report mods in callbacks, and item parentage.

This makes two complementary improvements to the ParseCallbacks.
The first is that Mods are now announced, as a new type of
DiscoveredItem. The second is that the parentage of each item is
announced. The parent of an item is often a mod (i.e. a
C++ namespace) but not necessarily - it might be a struct within
a struct, or similar.

The reported information here is dependent on two pre-existing
bindgen options:
* whether to report C++ namespaces at all
* whether to report inline namespaces conservatively.

For that reason, the test suite gains two new tests.

Part of google/autocxx#124

bindgen-tests/tests/parse_callbacks/item_discovery_callback/header_item_discovery_with_namespaces.hpp

@@ -0,0 +1,30 @@
+void a();
+
+namespace B {
+    void c();
+
+    namespace D {
+        void e();
+    }
+
+    // We should not report empty namespaces
+    namespace F {
+    }
+
+    namespace {
+        void g();
+    }
+
+    inline namespace H {
+        void i();
+        namespace J {
+            void k();
+        }
+    }
+
+    struct L {
+        struct M {
+
+        };
+    };
+};