DwarfUnit: Rename "Node" to "CUNode" and propagate it through DwarfTypeUnit as well.

Since we'll now also need the split dwarf file name along with the
language in DwarfTypeUnits, just use the whole DICompileUnit rather than
explicitly handling each field needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198842 91177308-0d34-0410-b5e6-96231b3b80d8