[DWARFDebugLine] Use truncating data extractors for prologue parsing

Summary:
This makes the code easier to reason about, as it will behave the same
way regardless of whether there is any more data coming after the
presumed end of the prologue.

Reviewers: jhenderson, dblaikie, probinson, ikudrin

Subscribers: hiraditya, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77557