[flang][openmp] Complex function returns and arguments error when offloading to NVPTX even when not used on device

Compiling

```fortran
function complex_add(a,b)
complex, value :: a, b
complex :: complex_add
complex_add = a + b
end
```

with these arguments

```
$ flang -fopenmp --offload-arch=sm_89 -c complex_return.f90
```

leads to this error

```
loc("/home/acc/Documents/flang_root/llvm-project/complex_return.f90":1:1): error: /home/acc/Documents/flang_root/llvm-project/flang/lib/Optimizer/CodeGen/Target.cpp:1525: not yet implemented: handle complex return types
LLVM ERROR: aborting
```

I did a bisect and found that this started happening after a5bb77f7e1240f7b06937578cc1c0c9d29e79e98. Looks like removing host-only functions later causes the todo to be hit when it was previously not.

Reproduce the failure with the shown complex_return.f90 source and the flang -fopenmp --offload-arch=sm_89 command. Read flang/lib/Optimizer/CodeGen/Target.cpp around line 1525 and inspect commit a5bb77f7e1240f7b06937578cc1c0c9d29e79e98, focusing on host-only function removal. Done means this host-only complex function no longer triggers the unimplemented complex return error during offloading.

Written by the indexing model from the issue text.