replace more EXP_XXXX_INTERPRET with ExpXxxxx

Part 3 of upgrading CTFE; this one fixes the special EXP values for statement control flow.
You could do something like this.
```cpp
class CTFEExp : public Expression
{
   CTFEExp(TOK tok)
    : Expression(Loc(0), tok, sizeof(CTFEExp))
    {}

    static CTFEExp* voidExp = new CTFEExp(TOKvoidexp);
    static CTFEExp* breakExp = new CTFEExp(TOKbreak);
}
```