Have the skeleton compile unit construction method take the CU it
is constructing from as an input and keep the same unique identifier.
We can use this to connect items which must stay in the .o file
(e.g. pubnames and pubtypes) to the skeleton cu rather than having
duplicate unique numbers for the sections and needing to do lookups
based on MDNode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189293 91177308-0d34-0410-b5e6-96231b3b80d8