Reinstate DW_AT_comp_dir support after D55519.

The DIFile used by the CU is special and distinct from the main source
file. Its directory part specifies what becomes the DW_AT_comp_dir
(the compilation directory), even if the source file was specified
with an absolute path.

To support the .dwo workflow, a valid DW_AT_comp_dir is necessary even
if source files were specified with an absolute path.

llvm-svn: 349065