Fix validation of provides lists of object literals.

[mfelsche]

Previously using a type alias pointing to a UNION type in the provides list of an object literal made the compiler assert.

fixes #2829

There is still a missing case where one would in general expect a compiler error, but nothing is thrown: Having a union of subtypes behind a type-alias:

interface I1
interface I2

type T1 is (I1 | I2)

actor Main
  new env(e: Env) =>
    object is T1 end

By the time the object AST is resolved the union folding already kicked in, so T1 only resolves to I1, not a union anymore. I am unsure what to do about this. Is this really a case that we should care about?

[mfelsche]

We do have a segfault on armhf running the testsuite, restarting the job.