Variable access to a local struct

After #551, I've found the following example of broken behavior:
```
int main(void) {
struct foo {
int x;
} bar;
bar.x = 42; // Type Cesium.CodeGen.Ir.Types.StructType was not found.
return bar.x;
}
```

It may be fixed by rolling back commit baa2fde359087c82875b4a6e4709d8b5f93274e8 (in particular, see changes in `LValueInstanceField`), but I currently feel the whole type gen process should be rearchitectured.

So, for now this will have to be broken until we do #201.

Reproduce the local-struct example and inspect the changes to LValueInstanceField in commit baa2fde359087c82875b4a6e4709d8b5f93274e8. Read the related work in issue #201 before deciding on an approach, since this issue says the type-generation process may need rearchitecting. Done means the example no longer reports that StructType was not found and variable access works.

Written by the indexing model from the issue text.