Incorrect span for mismatched type error during FRU

In the following example, foo is bool when it's expected to be int.

```
auto a = rec(foo = 0);
auto b = rec(foo = true with a);
```

The mismatched type error spans the entire rec being assigned to variable b, instead of just the incorrect expression, 'true'. For errors that occur in large records this can get pretty confusing.
