Spirv-Val Does Not Validate Against Empty Structs

Upstream issue https://github.com/shader-slang/slang/issues/5351. You can see the problematic spirv includes a `%5 = OpTypeStruct` which is an OpTypeStruct with no arguments. This causes nvidia's drivers to segfault and causes mesa to outright the reject the pipeline.

As far as I can tell this is indeed invalid, so it would be nice to get validation against this, as it went unnoticed.

Start from the reported `%5 = OpTypeStruct` with no members and compare it with the validation path for OpTypeStruct; use the linked upstream issue as reproducer context. Add a regression case showing the construct is rejected, and confirm the validator reports an error instead of accepting it.

Written by the indexing model from the issue text.