Ignore DIEs in the skeleton unit in a DWO scenario

Summary:
r362103 exposed a bug, where we could read incorrect data if a skeleton
unit contained more than the single unit DIE. Clang emits these kinds of
units with -fsplit-dwarf-inlining (which is also the default).

Changing lldb to handle these DIEs is nontrivial, as we'd have to change
the UID encoding logic to be able to reference these DIEs, and fix up
various places which are assuming that all DIEs come from the separate
compile unit.

However, it turns out this is not necessary, as the DWO unit contains
all the information that the skeleton unit does. So, this patch just
skips parsing the extra DIEs if we have successfully found the DWO file.
This enforces the invariant that the rest of the code is already
operating under.

This patch fixes a couple of existing tests, but I've also included a
simpler test which does not depend on execution of binaries, and would
have helped us in catching this sooner.

Reviewers: clayborg, JDevlieghere, aprantl

Subscribers: probinson, dblaikie, lldb-commits

Differential Revision: https://reviews.llvm.org/D62852

llvm-svn: 362586