Replace non-recursive sys::Mutex users with std::mutex
Replace non-recursive sys::Mutex users with std::mutex

Also remove a use of sys::MutexImpl, that's just evil. No functionality
change intended.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@368157 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm