[DWARFDebugLine] Use new DWARFDataExtractor::getInitialLength

Summary:
The error messages change somewhat, but I believe the overall
informational value remains unchanged.

Reviewers: jhenderson, dblaikie, ikudrin

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75116