Make sure that we always get the underlying concrete type or the anchor
of a particular equivalence class.

We’re substituting in the conformance type for Self, nothing more.

If substitution into the generic parameters of a generic function type
changes those generic parameters, build a new generic signature from
the resulting parameters. Otherwise, we can end up building ill-formed
generic signatures that fail validation.

When we canonicalize a generic signature, ensure that the resulting
signature has canonical requirements in the appropriate order.
More validation for generic signature canonicalization as part of the
ABI, which is tracked by SR-3733 / rdar://problem/31412994.