fix Issue 10573 - Weird linking problem with associative array cast [DMD 2.63]

CastExp would call `TypeAArray::getImpl()` in glue layer.

However if the AA type is just only used in the cast expression, the timing is too late to invoke `getImpl`, because the AA type instantiation may need more semantic analysis like deferred semantic3.

For the correct instantiation of the `AssociativeArray!(K, V)` type, `getImpl` should be invoked in front-end layer beforehand.