Adds a diagnostic when the user annotates an if constexpr with a likelihood attribute. The if constexpr statement is evaluated at compile time so the attribute has no effect. Annotating the accompanied else with a likelihood attribute has the same effect as annotating a generic statement. Since the attribute there is most likely not intended, a diagnostic will be issued. Since the attributes can't conflict, the "conflict" won't be diagnosed for an if constexpr.
It extends diagnostics on a patch in review.
Depends on D89899