Thread Safety Analysis: add a -Wthread-safety-negative flag that warns whenever
a mutex is acquired, but corresponding mutex is not provably not-held.  This
is based on the earlier negative requirements patch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214789 91177308-0d34-0410-b5e6-96231b3b80d8