Variable access to a local struct

[ForNeVeR]

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 baa2fde (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.