fix an invariance bug in limit_type_depth. part of #23786

[JeffBezanson]

This fixes the specific issue reported, but after this I get a SIGILL in the package's tests. Running just the last testset gives

broadcast! matching indices: Error During Test
  Got an exception of type MethodError outside of a @test
  MethodError: Base.Broadcast.promote_containertype(::Type{TypeSortedCollections.TypeSortedCollection}, ::Type{TypeSortedCollections.TypeSortedCollection}) is ambiguous. Candidates:
    promote_containertype(::Type{TypeSortedCollections.TypeSortedCollection}, _) in TypeSortedCollections at /home/jeff/.julia/v0.7/TypeSortedCollections/src/TypeSortedCollections.jl:199
    promote_containertype(_, ::Type{TypeSortedCollections.TypeSortedCollection}) in TypeSortedCollections at /home/jeff/.julia/v0.7/TypeSortedCollections/src/TypeSortedCollections.jl:200
    promote_containertype(::Type{T}, ::Type{T}) where T in Base.Broadcast at broadcast.jl:43
  Possible fix, define
    promote_containertype(::Type{TypeSortedCollections.TypeSortedCollection}, ::Type{TypeSortedCollections.TypeSortedCollection})

but with the full suite this turns into a SIGILL.

[tkoolen]

Hmm, that last test set also results in a method ambiguity error on 0.6 when run in isolation (and rightly so; a mistake on my part!), but passes when run after some other tests.

[ararslan]

I suspect my backporting of this to release-0.6 is not fully correct. Could you take a look, @JeffBezanson? 4a8d3a6