Sometimes an obligation depends on a later one, so we can't just process them in order like it was done previously.

This is not a problem in our test suite, but there may be ICEs out there and it will definitely be a problem with lazy TAIT.

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

Because MIPSr6 has many differences with previous MIPSr2 arch, the previous rlib metadata stripping code in `rustc_codegen_ssa` is only for MIPSr2/r3/r5 (which share the same elf e_flags).
This commit fixed this problem. It makes `rustc_codegen_ssa` happy when compiling rustc for MIPSr6 target or hosts.

It's already a (fat) reference.
Double referencing it creates lifetime issues for its methods that want to return iterators.

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.