Auto merge of #133929 - saethlin:remove-inline-in-all-cgus, r=nnethercote

Remove -Zinline-in-all-cgus and clean up tests/codegen-units/

Implementation of rust-lang/compiler-team#814

I've taken some liberties with cleaning up the CGU partitioning tests, because that's the only place this flag was used and also mattered. I've often fought a lot with the contents of `tests/codegen-units` and it has never been clear to me when a test failure indicates a problem with my changes as opposed to a test just needing to be manually blessed. Hopefully the combination of the new README, new comments, and using `-Zprint-mono-items=lazy` in the partitioning tests improves that.

I've also deleted some of the `tests/run-make/sepcomp` tests. I think all the "sepcomp" tests have been obviated for years by better-designed (less flaky, clearer failures) test suites, but here I'm just deleting the ones I'm confident in.

Author: bors