DebugInfo: Reuse existing LexicalScope to retrieve the scope's MDNode, rather than looking it up through the DebugLoc.

No functional change intended, just streamlines the abstract variable
lookup/construction to use a common entry point.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210234 91177308-0d34-0410-b5e6-96231b3b80d8