[clang] [llvm] [CIR][CUDA] Introduce cu.var_registration for shadow and attach device-side var metadata, internalize device side variables, lower poison attribute, introduce cu.var_registration attribute (PR #190087)

[Zaky Hermawan via cfe-commits]

https://github.com/ZakyHermawan edited https://github.com/llvm/llvm-project/pull/190087