[AMDGPU] HIP error: unsupported expression in static initializer: addrspacecast

The following HIP program triggers a compiler error during compilation. The compiler fails to handle an address space cast from shared memory (`addrspace(3)`) to a generic pointer within a static initialiser context.

**Test File:**
[HIPProg.hip](https://gist.github.com/ElliotBLabs/a75fbab5585b18d6ea5dd10c18f18333)

**How to reproduce:**
```bash
clang++ -x hip HIPProg.hip --offload-arch=native
```

**Actual Output:**
```text
error: unsupported expression in static initializer:
addrspacecast (ptr addrspace(3) @_ZZ6func_1P2S1E6l_1069 to ptr)
...
7 errors generated when compiling for gfx906.
```

I am not sure if this is just an ungraceful error or something not allowed in the generation.