[CUDA] Print an error if you try to compile with < sm_30 on CUDA 9.

Summary:
CUDA 9's minimum sm is sm_30.

Ideally we should also make sm_30 the default when compiling with CUDA
9, but that seems harder than it should be.

Subscribers: sanjoy

Differential Revision: https://reviews.llvm.org/D39109

llvm-svn: 316611