Use DWARFDataExtractor::getInitialLength in debug_aranges

Summary:
getInitialLength is a *DWARF*DataExtractor method so I had to "upgrade"
some DataExtractors to be able to make use of it.

Reviewers: ikudrin, jhenderson, probinson

Subscribers: aprantl, hiraditya, llvm-commits, dblaikie

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75535