Compiler asserts on concatenating a struct to an untypedlist

```c3 module main; struct Foo { int x, y; } fn void main() { untypedlist $list; $list +++= (Foo){1, 2}; } ``` Fails ``` ⚠️ The compiler encountered an unexpected error: "Should be unreachable". - Function: sema_expr_analyse_ct_concat(...) - Source file: /src/compiler/sema_const.c:436 ```