[DWARF] Use getInitialLength in range list parsing

Summary:
This could be considered obvious, but I am putting it up to illustrate
the usefulness/impact of the getInitialLength change.

Reviewers: dblaikie, jhenderson, ikudrin

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75117