[PATCH] Remove support for runtime multithreading

[Zachary Turner]

I tried this once and ran into some difficulty, because before this change,
the Mutex is a pointer which null is an expected value if
llvm_start_multithreaded has not been called.  So the function would have
to return a pointer to the mutex.  That would require changing MutexGuard
to take a pointer instead of a reference, which would generate a ton of
churn.

I'll have another stab at it and see if there's a way to do it in a way
that has very little churn and still contributes to this change as well.

http://reviews.llvm.org/D4139