These are not strictly unresolvable references. Treating them as such
fails an assertion in the `delete UnaryExpression` semantic (which is
Reference::delete_ in our implementation) - we enter the unresolvable,
branch, which then asserts that the [[Strict]] slot of the reference is
false.
This ensures the value goes through the regular ToPrimitive mechanism,
which PropertyKey::from_value() bypasses. This is relevant for objects
with a @@toPrimitive method, for example.
Also enables one skipped test in delete-basic.js, which now passes.
