[PATCH] Replace all standard mutexes with equivalent stl mutexes.

[Zachary Turner]

>>! In D4223#6, @chandlerc wrote:
> Any particular reason for universally using recursive_mutex? I feel like a lot of these would be pretty easily switched to mutex.

I actually had the same thought, but I felt that a secondary pass in a separate patch would be better for dealing with this.  The idea with this patch is to not change any behavior anywhere, only the mutex implementation.  So recursive mutexes are replaced with recursive mutexes, and non-recursive mutexes (which ironically there's only 1 of, and in a unittest no less) are replaced with non-recursive mutexes.

http://reviews.llvm.org/D4223