[DWARF] Use DWARFDataExtractor::getInitialLength to parse debug_names

Summary:
In this patch I've done a slightly bigger rewrite to also remove the
hardcoded header lengths.

Reviewers: jhenderson, dblaikie, ikudrin

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75119