[clang] [clang][SPIR][SPIRV] Don't generate constant NULL from addrspacecast generic NULL (PR #165353)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> OpenCL spec forbids casting between generic and constant address space pointers.

Doesn't really matter for the purposes of what the IR, it's a question of what the SPIRV machine allows 

https://github.com/llvm/llvm-project/pull/165353