[CUDA] Split device-var-init.cu tests into separate Sema and CodeGen parts.

Codegen tests for device-side variable initialization are subset of test
cases used to verify Sema's part of the job.
Including CodeGenCUDA/device-var-init.cu from SemaCUDA makes it easier to
keep both sides in sync.

Differential Revision: http://reviews.llvm.org/D20139

llvm-svn: 270107