Applying external functions with map builtin broken in interpreter

The following code used to work (float::unflatten is imported from another file):
```
#![test]
fn test_fft() {
...
let real_in: F32[4] = map(u32[4]:[1,2,3,4], float32::unflatten);
...
}
```
Now, this produces the following error:
```
[ RUN UNITTEST ] test_fft
E0315 20:27:23.176871 16 interpreter.cc:319] INTERNAL: XLS_RET_CHECK failure (xls/dslx/interpreter.cc:319) fn_data.function->owner() == current_type_info_->module() (0x55ee591558a0 vs. 0x55ee5914d0c0)
0x55ee55061d9a: absl::internal_statusor::StatusOrData<>::StatusOrData<>()
0x55ee550637f6: xls::dslx::Interpreter::CallFnValue()
0x55ee55063840: xls::dslx::AbstractInterpreterAdapter::CallValue()
0x55ee5507205b: xls::dslx::BuiltinMap()
0x55ee55061a83: xls::dslx::Interpreter::RunBuiltin()
0x55ee5506379e: xls::dslx::Interpreter::CallFnValue()
0x55ee550682e3: xls::dslx::Interpreter::EvaluateInvocation()
0x55ee55068c7e: xls::dslx::Evaluator::HandleInvocation()
0x55ee55061f43: xls::dslx::Interpreter::Evaluate()
0x55ee5506238d: xls::dslx::AbstractInterpreterAdapter::Eval()
0x55ee5508bac8: xls::dslx::EvaluateLet()
0x55ee5505fd9b: xls::dslx::Evaluator::HandleLet()
0x55ee55061f43: xls::dslx::Interpreter::Evaluate()
0x55ee5506759f: xls::dslx::Interpreter::RunTest()
0x55ee54fcfecb: xls::dslx::(anonymous namespace)::ParseAndTest()
0x55ee54f687d5: main
0x7f91e44230b3: __libc_start_main

E0315 20:27:23.176933 16 interpreter.cc:179] error @ xls/third_party/machsuite/fft/fft.x:136:28-136:66: INTERNAL: XLS_RET_CHECK failure (xls/dslx/interpreter.cc:319) fn_data.function->owner() == current_type_info_->module() (0x55ee591558a0 vs. 0x55ee5914d0c0)
E0315 20:27:23.176946 16 interpreter.cc:179] error @ xls/third_party/machsuite/fft/fft.x:136:3-159:1: INTERNAL: XLS_RET_CHECK failure (xls/dslx/interpreter.cc:319) fn_data.function->owner() == current_type_info_->module() (0x55ee591558a0 vs. 0x55ee5914d0c0)
E0315 20:27:23.176983 16 interpreter_main.cc:153] Internal error: INTERNAL: XLS_RET_CHECK failure (xls/dslx/interpreter.cc:319) fn_data.function->owner() == current_type_info_->module() (0x55ee591558a0 vs. 0x55ee5914d0c0)
[ FAILED ] test_fft: internal error: INTERNAL: XLS_RET_CHECK failure (xls/dslx/interpreter.cc:319) fn_data.function->owner() == current_type_info_->module() (0x55ee591558a0 vs. 0x55ee5914d0c0)
```

The issue goes away if we add a local function that calls float32::unflatten:
```
fn my_unflatten(myin: u32) -> F32 {
float32::unflatten(myin)
}

#![test]
fn test_fft() {
...
let real_in: F32[4] = map(u32[4]:[1,2,3,4], my_unflatten);
...
}
```