Fix assignment to `where` creating unintentional functions (#62067)
Fix #62051. Assigning to `where` (unless it's a function definition) is
generally disallowed, and assigning to a tuple in general is used for
destructuring rather than function definition, so this behaviour doesn't
make sense. It's also inconsistent with what lowering generally thinks is
a function definition (e.g. `global` wrapping this assignment is
broken).