Auto merge of #35718 - michaelwoerister:incr-comp-dir-locking, r=alexcrichton

Implement synchronization scheme for incr. comp. directory

This PR implements a copy-on-write-based synchronization scheme for the incremental compilation cache directory. For technical details, see the documentation at the beginning of `rustc_incremental/persist/fs.rs`.

The PR contains unit tests for some functions but for testing whether the scheme properly handles races, a more elaborate test setup would be needed. It would probably involve a small tool that allows to manipulate the incremental compilation directory in a controlled way and then letting a compiler instance run against directories in different states. I don't know if it's worth the trouble of adding another test category to `compiletest`, but I'd be happy to do so.

Fixes #32754
Fixes #34957

diff --cc src/librustc_driver/driver.rs
Simple merge