Fix lowering of associated types and synthesis of dispatch functions. (#4568)

* Treat global variables and parameters as non-differentiable when checking derivative data-flow

Global parameters are by-default not differentiable (even if they are of a differentiable type), because our auto-diff passes do not touch anything outside of function bodies.

The solution is to use wrapper objects with differentiable getter/setter methods (and we should provide a few such objects in the stdlib).

Fixes: #3289

This is a potentially breaking change: User code that was previously working with global variables of a differentiable type will now throw an error (previously the gradient would be dropped without warning). The solution is to use `detach()` to keep same behavior as before or rewrite the access using differentiable getter/setter methods.

* Fix issues with lookup witness lowering

* Update slang-ir-lower-witness-lookup.cpp

* Add tests

* Update slang-ir-lower-witness-lookup.cpp

* Cleanup

* Update nested-assoc-types.slang

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Co-authored-by: Yong He <yonghe@outlook.com>

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