[lldb/DWARF] Don't automatically search dwo unit attributes

[lldb/DWARF] Don't automatically search dwo unit attributes

This patch removes the code (deep inside DWARFDebugInfoEntry) which
automagically returned the attributes of the dwo unit DIE when asking
for the attributes of the skeleton unit.  This is fairly hacky, and not
consistent with how llvm DWARF parser operates.

Instead, I change the code the explicitly request (via
GetNonSkeletonUnit) the right unit to search (there were just two places
that needed this). If it turns out we need this more often, we can
create a utility function (external to DWARFUnit) for doing this.