[clang] [AMDGPU] Do not allow the region address space to be converted to generic (PR #117171)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> Should also forbid the buffers (probably should forbid using the buffers at all)

I think the logic currently allows 0, 1, 3, 4, and 5.

https://github.com/llvm/llvm-project/pull/117171