provide full type info when calling _aaGetX

This needs https://github.com/D-Programming-Language/druntime/pull/1037 to be merged first.

Precise scanning needs the full type info when creating Impl and other objects used by the AA implementation. This replaces the key type info with the AA typeinfo that contains both value and key type info.

It is calling _aaGetY instead of _aaGetX to allow the druntime PR to pass.