Ensure that XCVersionGroup always has a file type.

Without this, xcode doesn't accept the data model's version group.

--
MOS_MIGRATED_REVID=86796043

src/objc_tools/xcodegen/java/com/google/devtools/build/xcode/xcodegen/CurrentVersionSetter.java

@@ -83,9 +83,9 @@ private void trySetCurrentVersion(XCVersionGroup group) {
     }
 
     for (PBXFileReference child : group.getChildren()) {
+      child.setExplicitFileType(Optional.of("wrapper.xcdatamodel"));
       if (child.getName().equals(currentVersion.getContent())) {
         group.setCurrentVersion(Optional.of(child));
-        return;
       }
     }
   }