msvcrt: Forward ANSI open functions to their Unicode counterparts instead of the other way around. Implement _wfreopen.

Add compile-time asserts to check that the structures have the expected layout.

This was pointed out by Christoph von Wittich.

Use these to implement a few helper functions. Change the type
verification in type accessor functions to use these new functions.

widl: Add a new function, type_alias_get_aliasee to wrap  the retrieval of the type that the alias aliases.