Determine struct extensions before calling IRDiffDumper

This commit moves the functions that determine struct extensions from
ir_diff_representation.cpp to abi_diff_helpers.cpp. The functions
are called before IRDiffDumper so that the IR and the dumper do not
include the diff logic.

Test: ./test.py
Bug: 259148872
Change-Id: I93aaa3028bf6a30312f0b0e5b1ef5f6ae1041f21
5 files changed