[DSLX] Using types where we expect expressions causes internal error

**Describe the bug**

```
$ cat /tmp/simple.x
import apfloat;

type BF16 = apfloat::APFloat;
```

```
E0224 10:49:57.210159 1008868 command_line_utils.cc:47] Could not extract a textual position from error message: INTERNAL: XLS_RET_CHECK failure (xls/dslx/type_system/deduce.cc:1752) std::holds_alternative(eot) : INVALID_ARGUMENT: Provided status is not in recognized error form: INTERNAL: XLS_RET_CHECK failure (xls/dslx/type_system/deduce.cc:1752) std::holds_alternative(eot)
Error: INTERNAL: XLS_RET_CHECK failure (xls/dslx/type_system/deduce.cc:1752) std::holds_alternative(eot)
```

**To Reproduce**
Put the content above in a .x file and run `dslx_interpreter_main` on it with a DSLX standard library available.

**Expected behavior**
No internal error, good error message. Note this is likely to be fixed in TIv2 but we can make a test case for it regardless in the bridge time until TIv2 is the default.