Fix union layout when it contains 0 sized array.

Fix #3068

Author: Benjamin Lerman

bindgen-tests/tests/expectations/tests/union_with_zero_sized_array.rs

@@ -0,0 +1,5 @@
+union U {
+  char d0[0];
+  char d1[1];
+  char d2[2];
+};

bindgen-tests/tests/headers/union_with_zero_sized_array.h

@@ -266,7 +266,12 @@ impl<'a> StructLayoutTracker<'a> {
             }
         };
 
-        self.latest_offset += field_layout.size;
+        if !is_union {
+            self.latest_offset += field_layout.size;
+        } else {
+            self.latest_offset =
+                cmp::max(self.latest_offset, field_layout.size);
+        }
         self.latest_field_layout = Some(field_layout);
         self.max_field_align =
             cmp::max(self.max_field_align, field_layout.align);