[CUDA] Don't crash when trying to printf a non-scalar object.

Summary:
We can't do the right thing, since there's no right thing to do, but at
least we can not crash the compiler.

Reviewers: majnemer, rnk

Subscribers: cfe-commits, jhen, tra

Differential Revision: http://reviews.llvm.org/D17103

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260479 91177308-0d34-0410-b5e6-96231b3b80d8