DWARFDebugLoc: Make parsing and error reporting more robust

Summary:
While examining this class for possible use in lldb, I noticed two
things:
- it spits out parsing errors directly to stderr
- the loclists parser can incorrectly return valid location lists when
  parsing malformed (truncated) data

I improve the stderr situation by making the parseOneLocationList
functions return Expected<T>s. The errors are still dumped to stderr by
their callers, so this is only a partial fix, but it is enough for my
use case, as I intend to parse the locations lists one by one.

I fix the behavior in the truncated scenario by using the newly
introduced DataExtractor Cursor API.

I also add tests for handling the error cases, as they currently have no
coverage.

Reviewers: dblaikie, JDevlieghere, probinson

Subscribers: lldb-commits, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63591

llvm-svn: 370363