Diff the fields located at the same offset in a class

- Rewrite the diff algorithm to handle multiple fields located at the
  same offset in a class, struct or union.
- Allow renaming a field if its (offset, type) pair is unique.

Bug: 255702405
Test: ./test.py
Change-Id: I79ebe9a827822b0f89d95952fa7d8c30499ca680
8 files changed