Upstreaming: Promote cast.unsafe from a warning to an error

[cpovirk]

9ab109c

I assume that we don't really need to upstream this: The built-in CF nullness checker reports a full error for cases like casting from @Nullable Object to int. It might well always do this in cases that are actually unsafe (although then... why issue a warning?). (The built-in CF nullness checker also doesn't report an error, so perhaps it takes some action to short-circuit the cast.unsafe check.) Presumably we could do the same. But for now, I made cast.unsafe an error in our copy.

[cpovirk]

See also jspecify/jspecify-reference-checker@559ba69, in which we undo some of the errors created by this change... :)

[cpovirk]

If for some reason we need this in the long term, we could probably avoid carrying a patch against CF by overriding SourceChecker.reportWarning to behave like reportError.