[CUDA] Fix name of __activemask()

[Hahnfeld]

Summary

The name has two underscores in the official CUDA documentation:
http://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#warp-vote-functions

[jlebar]

Thank you for the fix!

cfe/trunk/lib/Headers/__clang_cuda_intrinsics.h

 inline __device__ int __uni_sync(unsigned int mask, int pred) {
   return __nvvm_vote_uni_sync(mask, pred);
 }
 
 inline __device__ unsigned int __ballot_sync(unsigned int mask, int pred) {
   return __nvvm_vote_ballot_sync(mask, pred);
 }
 
-inline __device__ unsigned int activemask() { return __nvvm_vote_ballot(1); }
+inline __device__ unsigned int __activemask() { return __nvvm_vote_ballot(1); }
 
 #endif // !defined(__CUDA_ARCH__) || __CUDA_ARCH__ >= 300
 
 // Define __match* builtins CUDA-9 headers expect to see.
 #if !defined(__CUDA_ARCH__) || __CUDA_ARCH__ >= 700
 inline __device__ unsigned int __match32_any_sync(unsigned int mask,
                                                   unsigned int value) {
   return __nvvm_match_any_sync_i32(mask, value);