[HLSL] Invalid struct field definition not producing error.

This struct definition should produce an error:
```
struct MoreBFields {
uint : G;
};
```

It is suspected that the G is being parsed as a semantic.
https://godbolt.org/z/5PfKenYzG

Start with the HLSL struct reproducer shown in the issue and compare its behavior with the linked Godbolt example. Trace how `uint : G;` is parsed, with the suspected semantic interpretation in mind; done means this invalid struct field produces an error.

Written by the indexing model from the issue text.