Rollup merge of #50538 - michaelwoerister:atomic-cnums, r=Zoxc

 Make CrateNum allocation more thread-safe.

This PR makes sure that we can't have race conditions when assigning CrateNums. It's a slight improvement but a larger refactoring of the CrateStore/CrateLoader infrastructure would be good, I think.

r? @Zoxc

diff --cc src/librustc_metadata/creader.rs
Simple merge

diff --cc src/librustc_metadata/cstore.rs
Simple merge