[REG2.064] Issue 11730 - associative array with Nullable!SysTime values: Called `get' on null Nullable!SysTime

https://d.puremagic.com/issues/show_bug.cgi?id=11730

When I fixed [bug 6178](https://d.puremagic.com/issues/show_bug.cgi?id=6178), I internally used `cast(void)exp)` to suppress unexpected "has no side effect" error. [here](https://github.com/D-Programming-Language/dmd/blob/117fc133f078de7bfb312d547378e32ca4f17b51/src/expression.c#L11623)

But, `CastExp` always try to resolve alias this of its operand to find most matching type to `void` via `Expression::castTo`. It should be disabled.