Fixes alignment errors with new Rust union type

This fix creates a new private field with the required aligned size. This new
private field ensures that the union has the required size.

Author: bkchr

src/codegen/mod.rs

@@ -1600,14 +1600,20 @@ impl CodeGenerator for CompInfo {
             );
         }
 
-        if is_union && !ctx.options().unstable_rust {
+        if is_union {
             let layout = layout.expect("Unable to get layout information?");
             let ty = BlobTyBuilder::new(layout).build();
-            let field = StructFieldBuilder::named("bindgen_union_field")
-                .pub_()
-                .build_ty(ty);
+            
+            let field = if ctx.options().unstable_rust {
+                StructFieldBuilder::named("_bindgen_union_align")
+                    .build_ty(ty)
+            } else {
+                struct_layout.saw_union(layout);
 
-            struct_layout.saw_union(layout);
+                StructFieldBuilder::named("bindgen_union_field")
+                    .pub_()
+                    .build_ty(ty)
+            };
 
             fields.push(field);
         }