[DWARF] Add support for parsing/dumping section indices in location lists

Summary:
This does exactly what it says on the box. The only small gotcha is the
section index computation for offset_pair entries, which can use either
the base address section, or the section from the offset_pair entry.
This is to support both the cases where the base address is relocated
(points to the base of the CU, typically), and the case where the base
address is a constant (typically zero) and relocations are on the
offsets themselves.

Reviewers: dblaikie, JDevlieghere, aprantl, SouraVX

Subscribers: hiraditya, llvm-commits, probinson

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70540