Fix #12695 : RangeIndex generated with pd.concat with ignore_index as True
- [x] closes #12695
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

Replaced generated indexes with RangeIndex, test still needed

tests! always start with the tests to make sure it fails first!
 - needs some tests (use the one from the original issue)
- whatsnew entry (bug fix)
 Thanks for the PR:) But I've included the fix in #12846 to refactor existing code. Pls lmk if anything.
