[Clang][NVPTX] Add NVPTX intrinsics and builtins for CUDA PTX redux.sync instructions

Adds NVPTX builtins and intrinsics for the CUDA PTX `redux.sync` instructions
for `sm_80` architecture or newer.

PTX ISA description of `redux.sync`:
https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#parallel-synchronization-and-communication-instructions-redux-sync



Authored-by: Steffen Larsen <steffen.larsen@codeplay.com>

Differential Revision: https://reviews.llvm.org/D100124