fix Issue 11614 - Error: this for _expand_field_0 needs to be type Tuple not type Foo

Even if a variable has no valid context, it would be either:

1. a part of compile time evaluated expression, which is used as unreal context.
    enum x = Type.fieldvar.sizeof;
    // It's legitimate expression so fieldvar is not evaluated
    // in runtime.

2. or an invalid field access without valid 'this', in runtime evaluated expression.
    int y = Type.fieldvar;
    // It's invalid expression so fieldvar access has no valid 'this'.

For the case #2, it will be an error by `checkRightThis` later. Therefore `getRightThis` should not make it an error.