DataflowWorklist.h: Appease msc17 -- Split a private constructor to DataflowWorklistBase.

MSC17 isn't capable to invoke other constructors in its scope.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214067 91177308-0d34-0410-b5e6-96231b3b80d8