Thread Safety Analysis: support adopting of locks, as implemented in

Thread Safety Analysis: support adopting of locks, as implemented in
std::lock_guard.  If EXCLUSIVE_LOCKS_REQUIRED is placed on the constructor of
a SCOPED_LOCKABLE class, then that constructor is assumed to adopt the lock;
e.g. the lock must be held on construction, and will be released on destruction.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@228194 91177308-0d34-0410-b5e6-96231b3b80d8