[clang] Reapply "[Clang] Optionally use NewPM to run CodeGen Pipeline" (#205943) (PR #205986)

[Nikita Popov via cfe-commits]

nikic wrote:

This adds +5% to the time to build clang. Not entirely sure why. https://llvm-compile-time-tracker.com/compare_clang.php?from=d6d6f4f3c104f7a741ba96fca1bcb0a0039493f0&to=28f6605b844cabf8fa16201e4d3226fe3970d2f4&stat=instructions%3Au&sortBy=absolute-difference shows that a lot of CodeGen files become much slower to compile. But you did not add any includes in headers.

Maybe linking against CodeGen somehow changes which PCH we use to be less profitable? cc @aengelke 

https://github.com/llvm/llvm-project/pull/205986