Remove internals for typedef declaration

Supplemental PR for the change: https://github.com/D-Programming-Language/dmd/pull/4029

This PR also make the syntax `is(T == typedef)` an error. To do it, this PR needs corresponding Phobos fix:
https://github.com/D-Programming-Language/phobos/pull/2577
Not a deprecation, but an error. This fits in with the `case TOKtypedef` in expression.c being an assert now.