We previously used a (pretty bad) hack for WMO. This resulted in issues like #92.
Upstream now has proper support for WMO (see generally, https://github.com/apple/swift-llbuild/pull/28, https://bugs.swift.org/browse/SR-881).
We now use the upstream feature to handle this case. We also add -num-threads support, which upstream recently added.
Note that our implementation now only works for swift-DEVELOPMENT-SNAPSHOT-2016-05-09-a and above.