Simplify the return value of CompareCommonRecordFields

Before this commit, the function returns
(kDirectDiff, RecordFieldDiffIR *) or (kIndirectDiff/kNoDiff, nullptr).
The commit changes the return value to a DiffStatus. The caller decides
whether to create a RecordFieldDiffIR by the DiffStatus.

Test: ./test.py
Bug: 255702405
Change-Id: If2bfd5bca78e9d924be34ad5c522db9b91382cde
2 files changed