This is a backwards-compatible language change.

Previously, `@intToEnum` coerced its integer operand to the integer tag
type of the destination enum type, often requiring the callsite to
additionally wrap the operand in an `@intCast`. Now, the `@intCast` is
implicit, and any integer operand can be passed to `@intToEnum`.

The same as before, it is illegal behavior to pass any integer which does
not have a corresponding enum tag.