[clang][OpenCL][CodeGen][AMDGPU] Do not use `private` as the default AS for when `generic` is available (#112442)
Currently, for AMDGPU, when compiling for OpenCL, we unconditionally use
`private` as the default address space. This is wrong for cases where
the `generic` address space is available, and is corrected via this
patch. In general, this AS map abuse is a bad hack and we should re-work
it altogether, but at least after this patch we will stop being
incorrect for e.g. OpenCL 2.0.