DwarfUnit: Provide a reference to a defining DwarfCompileUnit from DwarfTypeUnit.

Type units need to insert their file strings into the compile unit's
line/file table. This is preliminary work to that end.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201196 91177308-0d34-0410-b5e6-96231b3b80d8