[CUDA, NVPTX] Added basic __bf16 support for NVPTX.

Recent Clang changes expose _bf16 types for SSE2-enabled host compilations and
that makes those types visible furing GPU-side compilation, where it currently
fails with Sema complaining that __bf16 is not supported.

Considering that __bf16 is a storage-only type, enabling it for NVPTX if it's
enabled on the host should pose no issues, correctness-wise.

Recent NVIDIA GPUs have introduced bf16 support, so we'll likely grow better
support for __bf16 on NVPTX going forward.

Differential Revision: https://reviews.llvm.org/D136311