The new warnings API is JS::WarnUTF8(), JS::WarnLatin1(), and
JS::WarnASCII(). It can now fail, in the case where the warning was
upgraded into an error.

There was one instance where we used the old ASCII API which is now
replaced by the UTF8 API. It is possible, though extremely unlikely,
that a boxed union type might have a non-ASCII name, so just change it
to UTF8 here.