[NVVM] Rename __nvvm_bar0 builtin back to __syncthreads.

__syncthreads was renamed to __nvvm_bar0 in r274664.  But __syncthreads
is part of our user-facing API, so we need to keep the name.

This will momentarily break clang; we need a matching patch there.

Patch by Justin Bogner.

llvm-svn: 274779