[clang] [clang][deps] Parallelize module compilations (PR #180047)

[Qiongsi Wu via cfe-commits]

https://github.com/qiongsiwu approved this pull request.

LGTM! Note that this change currently does not change the by-name lookup code path. I discussed this with @jansvoboda11 offline. We'd like to land a minimal change for now, and then consolidate the code paths later. @jansvoboda11 will add a FIXME so we don't forget. Thanks Jan! 

https://github.com/llvm/llvm-project/pull/180047