[llvm] [AMDGPU][LowerModuleLDS] Avoid crash on zero-sized types (PR #84742)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I just added a fatal error for now while we figure this out. I think it's better to just avoid crashing in the meantime.

If this appeared in the wild, making this an error is a lateral move. I don't think there's anything to figure out. Just ignore adding it to the OptimizedStructLayout, record there was a 0 sized object, and then handle it along with the external case 

https://github.com/llvm/llvm-project/pull/84742