HLSL/reflection: flattening/splitting loses semantic

input "t.UV" doesn't have the TEXCOORD0 semantic qualifier

```
struct S2 {
float2 UV;
};

struct S {
S2 t : TEXCOORD0;
};

S main(S ins)
{
S s = ins;
return s;
}
```

Start by reproducing the HLSL example and inspect how reflection handles flattening and splitting nested structures. Verify why input "t.UV" loses the TEXCOORD0 semantic qualifier, and consider the issue complete when that qualifier is preserved.

Written by the indexing model from the issue text.