Untangle filename/dirname confusion. Store constructed strings on the side....

Untangle filename/dirname confusion. Store constructed strings on the side. Avoid use of Path.makeAbsolute().

DW_TAG_compile_unit uses two attributes DW_AT_name and DW_AT_comp_dir. Their expected values are:

$ clang foo.c -g
  DW_AT_name - foo.c
  DW_AT_comp_dir - `pwd`

$ clang one/two/foo.c -g
  DW_AT_name - one/two/foo.c
  DW_AT_comp_dir - `pwd`

$ clang /tmp/one/foo.c -g
  DW_AT_name - /tmp/one/foo.c
  DW_AT_comp_dir - empty

llvm-svn: 109303