DebugInfo: Use the comp_dir of the referencing type units when building debug_line.dwo

This isn't a complete fix - it falls back to non-comp_dir when multiple
compile units are in play. Adding a map of comp_dir to table is part of
the more general solution, but I gave up (in the short term) when I
realized I'd also have to calculate the size of each type unit so as to
produce correct DW_AT_stmt_list attributes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204202 91177308-0d34-0410-b5e6-96231b3b80d8