DWARFDebugLoclists: Add an api to get the location lists of a DWARF unit

Summary:
This avoid the need to duplicate the location lists searching logic in
various users. The "inline location list dumping" code (which is the
only user actually updated to handle DWARF v5 location lists)  is
switched to this method. After adding v4 location list support, I'll
switch other users too.

Reviewers: dblaikie, probinson, JDevlieghere, aprantl, SouraVX

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70084