Allow appending members to structs and classes

This commit adds RecordTypeDiffIR::IsExtended() that determines whether
the difference is a pure extension, i.e., appending members and
increasing the size. The extensions are stored in
record_type_extension_diffs in abi_diff messages. They can be ignored by
specifying -allow-extensions.

Test: ./test.py
Bug: 248418092
Change-Id: Ibdef042c6176dc84d2dd61f71a3f88035e943dd4
14 files changed