DwarfUnit: Include type unit's file strings in the defining compile unit's file_names table

There's still one piece missing here, which is adding the
DW_AT_stmt_list to the type unit that refer's to the compile unit's line
table. Working on that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201198 91177308-0d34-0410-b5e6-96231b3b80d8