Add support for a type variable as a potentially constant expression

[eernstg]

This PR adds an extra kind of potentially constant expression, namely a type variable (which is necessarily declared by the enclosing class).

It is needed for consistency, because this is a semantics that we can already express using different mechanisms:

typedef F<X> = X;

class A<Y> {
  Type type;
  const A(): type = F<Y>; // Might as well allow `type = Y`.
}

The PR also moves one more kind of constant to the section about constant expressions, such that this section contains every kind of constant / potentially constant expression in this document which is not derived directly from the corresponding constructor function.

The ones that are based on the corresponding constructor function remain where they are, because it would duplicate a lot of material to refer to the details of the declaration of said functions in the constant expression section. The commentary that creates the connection has been updated accordingly.

[eernstg]

Review response.