Simplify `GetName`+`AppendTypeName` by `DWARFDIE`

In D61502#1503247 @clayborg suggested that DWARFUnit *+dw_offset_t can be now
replaced by DWARFDIE.

It is moved from DWARFDebugInfoEntry to DWARFDIE as noted by @clayborg.

I have also removed return type as (1) it was wrong in one case and (2) no
existing caller used the return type. I also refactored the deep nesting noted
by @JDevlieghere.

Differential Revision: https://reviews.llvm.org/D62211

llvm-svn: 361463