[Sema] Diagnose annotating `if constexpr` with a likelihood attribute

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`.

Differential Revision: https://reviews.llvm.org/D90336