Handle "ThisType" subsitutions when specialization generics in the IR

The original code is handling the issue where a call site might be specializing a generic function, so it has a `DeclRef` that represents what it wants to specialize, but the callee is actually a different overload of the same generic function (e.g., a target-specific overload) and so we need to construct a set of substitutions that are equivalent (same arguments), but point to different `GenericDecl`s.

That code was making some bad assumptions, though:

1. It assumed that the substitutions list would always start with a generic substitution (no longer true with `ThisTypeSubstitution`.
2. It assumed that only the top-most substitution would need to be translated. This assumption is probably safe for now, but it could break down if we ever introduced an ability for a type to be re-opened to introduce new (target-specific) overloads of its members.

The new approach goes ahead and does a deep copy of the substitition list (but a shallow copy of the arguments), and only copies the generic substititions for now.

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