[Clang][NVPTX]Add NVPTX intrinsics and builtins for CUDA PTX cvt sm80 instructions

Adds NVPTX intrinsics and builtins for CUDA PTX cvt instructions for sm80
architectures and above. Requires ptx 7.0.

PTX ISA description of cvt instructions :
https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#data-movement-and-conversion-instructions-cvt



Signed-off-by: JackAKirk <jack.kirk@codeplay.com>

Differential Revision: https://reviews.llvm.org/D116673