[ThinLTO] Detect partially split modules during the thin link

Summary:
The changes to disable LTO unit splitting by default (r350949) and
detect inconsistently split LTO units (r350948) are causing some crashes
when the inconsistency is detected in multiple threads simultaneously.
Fix that by having the code always look for the inconsistently split
LTO units during the thin link, by checking for the presence of type
tests recorded in the summaries.

Modify test added in r350948 to remove single threading required to fix
a bot failure due to this issue (and some debugging options added in the
process of diagnosing it).

Reviewers: pcc

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57561

Change-Id: I1df30bf57e0fb8b23a2444e07f1e0a04eb8ef2ea
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354062 91177308-0d34-0410-b5e6-96231b3b80d8