[CUDA] Don't try to run sanitizers on NVPTX.

Summary:
Sanitizers aren't supported on NVPTX -- don't try to run them.

This lets you e.g. pass -fsanitize=address and get asan on your host
code.

Reviewers: kcc

Subscribers: cfe-commits, tra, jhen

Differential Revision: https://reviews.llvm.org/D24640

llvm-svn: 281680