clang

Add pad kernel c++ for opencl #453

Job	Run time
linux-x64-clang	6m 48s
linux-x64-clang-constexpr-eval	5m 7s
linux-x64-clang-constexpr-no-stl-generic-ndarray-eval	8m 39s
linux-x64-clang-constexpr-generic-ndarray-eval	5m 56s
linux-x64-clang-index	4m 16s
linux-x64-clang-generic-ndarray	11m 14s
linux-x64-clang-generic-ndarray-eval	8m 35s
linux-x64-clang-index	1m 50s
linux-x64-clang-no-stl	4m 44s
linux-x64-clang-no-stl-generic-ndarray	9m 15s
linux-x64-clang-no-stl-generic-ndarray-eval	8m 50s
	1h 15m 14s

Provide feedback