DebugInfo: Sink DwarfDebug::ScopeVariables down into DwarfFile

(part of refactoring to allow subprogram emission in both the skeleton
and main units to enable -gmlt-like data to be included in the skeleton
for live inlined backtracing purposes)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220578 91177308-0d34-0410-b5e6-96231b3b80d8