The previous code assumed that each compile unit had its own abbreviation section, and they are all in order. That's normally how LLVM emits things, but in #2992 there is a testcase in which linking of object files with IR files somehow ends up with a different order. The proper fix is to track the binary offsets of abbreviations in the abbreviation section. That section is comprised of null-terminated lists, which each CU has an offset to the beginning of. With those offsets, we can match things properly. Add a testcase that crashes without this, to prevent regressions. Fixes #2992 Fixes #3007