[lib/Sema] Suggest `return` when the last statement would be a valid … (#42415)
* [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.