[flang][NFC] Address reported "possible missing return" (#86523)

A function uses "if constexpr" to consider all possible types in a
variant, but looks as if it can fall out without returning an
expression. Add a final "else" with a crash to make things more clear
and to protect against unlikely future extensions of the type.

Fixes https://github.com/llvm/llvm-project/issues/86391.