The SEH unwind info problem no longer happens because nowadays bug 1631929 prevents the code pattern that led to it. I've confirmed that bug 1626951's bustage doesn't come back, both in regular and beta-simulation builds.
This cleanup will allow me to reverse the dependency in bug 1677742 and have `lto` depend on `new_pass_manager_flags`.
Differential Revision: https://phabricator.services.mozilla.com/D97362