Relevant lines pasted from the grammar:
```c
    //G is-as-expression:
    //G     prefix-expression
    //G     is-as-expression is-value-constraint
    //GTODO     type-id is-type-constraint
```
A *prefix-expression* can be an *unqualified-id*, which can also be a
*type-id*. So when the latter is implemented, it would break any code
using `identifier is` to test an expression.

I understand the usual way of disambiguating is to require parens, so I
have done that here to fix 5 `regression_tests`.

Co-authored-by: Johel Ernesto Guerrero Peña <[email protected]>
Signed-off-by: Nick Treleaven <[email protected]>