Bad error message when using bits type in place of struct type

No reference is given to the DSLX source code.

```
fn foo() -> () {
let a = u15 { x: u32:42 };
()
}
```

```
$ bazel-bin/third_party/xls/dslx/interpreter_main input.x
...
E0113 15:44:24.360973 8501 deduce.cc:1765] INTERNAL: XLS_RET_CHECK failure (third_party/xls/dslx/deduce.cc:1765) struct_type != nullptr uN[16]
...
```