[OpenMP][NVPTX][CUDA] Adding support for printf for an NVPTX OpenMP device.

Support for CUDA printf is exploited to support printf for
an NVPTX OpenMP device.

To reflect the support of both programming models, the file
CGCUDABuiltin.cpp has been renamed to CGGPUBuiltin.cpp, and
the call EmitCUDADevicePrintfCallExpr has been renamed to
EmitGPUDevicePrintfCallExpr.

Reviewers: jlebar
Differential Revision: https://reviews.llvm.org/D17890

llvm-svn: 293444