[llvm-symbolizer] Fix parsing DW_AT_ranges in Fission skeleton compile unit DIEs.

[llvm-symbolizer] Fix parsing DW_AT_ranges in Fission skeleton compile unit DIEs.

Turns out that DW_AT_ranges_base attribute sets the offset for
DW_AT_ranges values specified in the .dwo file, but not for DW_AT_ranges specified
in the skeleton compile unit DIE in the main executable. This is extremely confusing,
and would hopefully be fixed in DWARF-5 when it's finalized. For now this
behavior makes sense, as otherwise Fission would break DWARF consumers who
doesn't know anything about DW_AT_ranges_base.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210809 91177308-0d34-0410-b5e6-96231b3b80d8