[CUDA][HIP] Mark device var used by host only

Add device variables to llvm.compiler.used if they are
ODR-used by either host or device functions.

This is necessary to prevent them from being
eliminated by whole-program optimization
where the compiler has no way to know a device
variable is used by some host code.

Reviewed by: Artem Belevich

Differential Revision: https://reviews.llvm.org/D98814