We had a strange relationship here where we made a list of Jobs
inherit from a single Job, but there weren't actually any places where
this arbitrary nesting was used or needed.

Simplify all of this by removing Job entirely and updating all of the
users to either work with a JobList or a single Command.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241310 91177308-0d34-0410-b5e6-96231b3b80d8