Auto merge of #68277 - michaelwoerister:re-export-dylib-instances, r=alexcrichton
Make sure that all upstream generics get re-exported from Rust dylibs.
This PR contains a fix for #67276. Rust dylibs would not re-export all generic instances when compiling with `-Zshare-generics=on` (=default for debug builds) which could lead to situations where the compiler expected certain generic instances to be available but then the linker would not find them.
### TODO
- [x] Write a regression test based on the description [here](https://github.com/rust-lang/rust/issues/67276#issuecomment-574613457).
- [x] Find out if this also fixes other issues related to https://github.com/rust-lang/rust/issues/64319.
r? @alexcrichton ~~(once the TODOs are done)~~
cc @pnkfelix @AlexKornitzer
