kcfi: only reify trait methods when dyn-compatible
fixes https://github.com/rust-lang/rust/issues/146853
Only generate a `ReifyShim` for trait method calls if the trait is dyn-compatible.
Until now kcfi would generate a `ReifyShim` whenever a trait method was cast to a function pointer. But technically the shim is only needed for dyn-compatible traits (where the method might end up in a vtable).
Up to this point that was only slightly inefficient, but in combination with c-variadic trait methods it is wrong. For c-variadic trait methods the generated shim is incorrect, and that is why c-variadic methods make a trait no longer dyn-compatible: we should simply never generate a `ReifyShim` that is c-variadic.
With this change the documentation on `ReifyReason` is now actually correct:
> If KCFI is enabled, creating a function pointer from a method on a dyn-compatible trait. This includes the case of converting `::call`-like methods on closure-likes to function pointers.
cc ```@maurer``` ```@workingjubilee```
r? ```@rcvalle```
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