[CUDA] Don't initialize the CUDA toolchain if we don't have any CUDA inputs.

Summary:
This prevents errors when you invoke clang with a flag that the NVPTX
toolchain doesn't support.  For example, on x86-64,

  clang -mthread-model single -x c++ /dev/null -o /dev/null

should output just one error about "invalid thread model 'single' in
'-mthread-model single' for this target"; x86-64 doesn't support
-mthread-model, but we shouldn't also instantiate a NVPTX target!

Reviewers: echristo

Subscribers: tra, sunfish, cfe-commits

Differential Revision: http://reviews.llvm.org/D18629

llvm-svn: 264965