* [lib/Sema] Suggest `return` when the last statement would be a valid return value
Addresses SR-10988. When a multi-statement function body contains no return keyword but the last statement would be a valid return value, the compiler currently proposes a fixit to add the return keyword.
This changes extends this behavior to functions with opaque return types.
* Rename --enable-experimental-opaque-return-types and gate structural opaque types with a flag
* Separate out structural opaque type result builder tests
* [TypeResolver][TypeChecker] Add support for structural opaque result types
* [TypeResolver][TypeChecker] Clean up changes that add structural opaque result types
