Since PassRegistry is currently a shared global object, it needs locking. While...

Since PassRegistry is currently a shared global object, it needs locking.  While it might intuitively seem
that all the setup of this class currently happens at static initialization time, this misses the fact
that some later events can cause mutation of the PassRegistrationListeners list, and thus cause race issues.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114036 91177308-0d34-0410-b5e6-96231b3b80d8