DWARFDebugNames: Fix lookup in dwo files

The getDIESectionOffset function is not correct for split dwarf files
(and will probably be removed in D48009).

This patch implements correct section offset computation for split and
non-split compile units -- we first need to check if the referenced unit
is a skeleton unit, and if it is, we add the die offset to the full unit
base offset (as the full unit is the one which contains the die).

llvm-svn: 334402