Fix discriminant caching issue

[ssalbdivad]

This tests passes in isolation but fails when run with the rest of the repo:

I've noticed that foo: "Function" incorrectly gets pruned to foo: "object" in this case but can't figure out where.

it("multiple switch", () => {
  const types = scope({
    a: { foo: "string" },
    b: { foo: "number" },
    c: { foo: "Function" },
    d: "a|b|c",
  }).export();
  // attest(types.d({}).toString()).snap(
  // 	"foo must be a number, an object or a string (was undefined)"
  // )
  // this could be improved, currently a bit counterintuitive because of
  // the inconsistency between `domainOf` and typeof
  attest(types.d({ foo: null }).toString()).snap(
    "foo must be a function (was null)"
  );
});