header-checker: Code cleanup

This commit breaks `HeaderAbiLinker::LinkAndDump()` into several
functions, moves `llvm::object::Binary`-related code to `SoFileParser`,
removes unused code in `IRReader`, and remove some space characters
before semicolon.

Test: ./tests/test.py
Change-Id: I9f90f07985c84ff95bf093c65ede091d44849e33