The official llvm 4.0.0 release is out, so next steps are to:

• pin the existing llvm 4.0.0 mason package to the release tarballs (currently pointed to git head)
• rebuild and republish 4.0.0
• create a 5.0.0 package that points at upstream llvm to begin allowing that to be tested

I'm out today, but will plan to hit this tomorrow.

/cc @kkaefer @daniel-j-h