DebugInfo: The rest of pushing DwarfDebug::constructScopeDIE down into DwarfCompileUnit

Funnily enough, I copied it, but didn't actually remove the original in
r219345. Let's do that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219346 91177308-0d34-0410-b5e6-96231b3b80d8