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
Needs to be followed by:
```
assert(0);
```
because this should never have gotten past the parser.