Add a lock() function in PassRegistry to speed up multi-thread synchronization.

When calling lock() after all passes are registered, the PassRegistry doesn't need a mutex anymore to look up passes.
This speeds up multithreaded llvm execution by ~5% (tested with 4 threads).
In an asserts build of llvm this has an even bigger impact.

Note that it's not required to use the lock function.

llvm-svn: 231276