[CUDA] Assume the latest known CUDA version if we've found an unknown one.

This makes clang somewhat forward-compatible with new CUDA releases
without having to patch it for every minor release without adding
any new function.

If an unknown version is found, clang issues a warning (can be disabled
with -Wno-cuda-unknown-version) and assumes that it has detected
the latest known version. CUDA releases are usually supersets
of older ones feature-wise, so it should be sufficient to keep
released clang versions working with minor CUDA updates without
having to upgrade clang, too.

Differential Revision: https://reviews.llvm.org/D73231