swift234

Reflection: Add TypeRef::deriveSubstitutions()

When deriving substitutions from closure contexts, we end up with
a problem where we have an original type and a substituted type,
and the original type is not necessarily a type parameter.
We need to decompose the original and substituted types to derive
the substitutions that produced the substitution.

For example, deriveSubstitutions(Foo<T -> Int>, Foo<String -> Int>)
will give us a substitution of T := Int.