[Clang] Change default triple to LLVM_HOST_TRIPLE for the CUDA toolchain

When cross-compiling NVPTX we use the triple to indicate which paths to
search for the CUDA toolchain. Currently this uses the default target
triple. This might not be exactly correct, as this is the default triple
used to compile binaries, not the host system. We want the host triple
because it indicates which folders should hold CUDA.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D150136