Generate getters/setters to contained anonymous struct/union members

We often see code like this:

struct JSJitInfo {
    // ...
    
    union {
        uint16_t protoID;
        js::jit::InlinableNative inlinableNative;
    };

    // ...
};

which generates members and types like __bindgen_anon_1: JSJitInfo__bindgen_ty_1.

It would be cool if we generated getters/setters on the containing struct for these anonymous structs/unions members, so that using them was easier and more like their usage in C/C++.

Start by locating how bindgen lowers anonymous struct and union members into _bindgen_anon* types. Define the expected getter and setter behavior on the containing struct, using the JSJitInfo example as a reference, then validate the generated bindings against an equivalent C++ declaration.