Use an RWMutex instead of a Mutex in PassRegistry

[Alex Crichton]

When using a LLVM's MCJIT from multiple threads, there was a huge
amount of contention in llvm::PassRegistry::getPassInfo. For the
particular suite of tests I was running, moving from a Mutex to an
RWMutex reduced the overall time from 6min to 20s. In both cases the
single-threaded case took about 40s (not all of this time is LLVM).

It basically seemed like it was a huge win for a small change, so yay!
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