DwarfCompileUnit: Push DIDescriptor usage out from isShareableAcrossCUs

This is the first of a few similar patches. We'll see how far it
goes/makes sense.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194871 91177308-0d34-0410-b5e6-96231b3b80d8