x/exp/apidiff: support type constraints

[pellared]

From #26420 (comment).

Adding a new type term to type element is not seen an API-level change.

For example when changing

type Number interface {
	~float32 | ~float64
}

to

type Number interface {
	~float32 | ~float64 | ~int
}

gorelease suggest to only bump the patch number.

I think it should bump minor number instead.

The reason is that apidiff does not know about type constraints.

[mateusz834]

I think it should bump minor number instead.

I wonder what that is correct in terms of backwards compatibility. Or to put it differently, in that case it is probably fine to only bump the minor version, but if you included a non-comparable type to that union, then (i believe) it is an incompatible change, since it could've been used in a map, for example:

type SomeConstraint interface {
	~float32 | ~float64 | ~[]byte // changed from ~float32 | ~float64
}

func test[T SomeConstraint]() {
	_ = map[T]struct{}{} // ERROR invalid map key type T (missing comparable constraint) [IncomparableMapKey]
}

There are for sure other cases like that.

[pellared]

@mateusz834, I agree with you