Fixtype of initializers [C:6.7.9].
This is a tagged union type, introduced by fty::deftagsum.
For now we only model initializers tht consists of either a single expression or a list of expressions, where the latter models a list between curly braces of initializers consisting of single expressions. Note that, since currently we do not model the comma operator, our use of any expressions here matches the use of assignment expressions in [C].