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

[cdleary]

Describe the bug

$ cat /tmp/simple.x 
import apfloat;

type BF16 = apfloat::APFloat<u8, u7>;

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<Expr*>(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<Expr*>(eot) 
Error: INTERNAL: XLS_RET_CHECK failure (xls/dslx/type_system/deduce.cc:1752) std::holds_alternative<Expr*>(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.