[CUDA] Make printf work.

Summary:
The code in CGCUDACall is largely based on a patch written by Eli
Bendersky:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20140324/210218.html

That patch implemented an LLVM pass lowering printf to vprintf; this
one does something similar, but in Clang codegen.

Reviewers: echristo

Subscribers: cfe-commits, jhen, tra, majnemer

Differential Revision: http://reviews.llvm.org/D16372

llvm-svn: 258642