[CUDA] Report "unsupported VLA" errors only on device side.

This fixes erroneously reported CUDA compilation errors
in host-side code during device-side compilation.

I've also restricted OpenMP-specific checks to trigger only
if we're compiling with OpenMP enabled.

Differential Revision: https://reviews.llvm.org/D40275

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@319201 91177308-0d34-0410-b5e6-96231b3b80d8