The issue here is that the logic used to determine which CGU to put the
dead function stubs in doesn't handle cases where a module is never
assigned to a CGU.

The partitioning logic also caused issues in rust-lang#85461 where inline
functions were duplicated into multiple CGUs resulting in duplicate
symbols.

This commit fixes the issue by removing the complex logic used to assign
dead code stubs to CGUs and replaces it with a much simplier model: we
pick one CGU to hold all the dead code stubs. We pick a CGU which has
exported items which increases the likelihood the linker won't throw
away our dead functions and we pick the smallest to minimize the impact
on compilation times for crates with very large CGUs.

Fixes rust-lang#86177
Fixes rust-lang#85718
Fixes rust-lang#79622

As discussed in
rust-lang#92142 (comment),
tests that contain multiple files order their results differently on
Windows and Linux which the test runner currently can't handle
correctly. For now, ignore the "bin" part of the test and only include
the unused library dependency which is what the test really cares about
anyway.