Empty llvm.func with linkonce_odr linkage.

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When running the following test:

```
// RUN: cgeist %s --function=* -llvm-struct=1 -memref-abi=0 -S | FileCheck %s

struct stream {
stream& operator<<(stream& (*__pf)(stream&)) { return __pf(*this); }
};

inline stream& endl(stream& stream)
{
return stream;
}

int main() {
stream str;
str << endl;
return 0;
}
```
I get the error message:

```error: 'llvm.func' op external functions must have 'external' or 'extern_weak' linkage```

The error message makes sense since there is an empty _Z4endlR6stream function that has linkonce_odr linkage.

On the Mac the example triggers also a slightly different error when I remove the inline keyword. In that case mlirs verifier complains bcs there is a redefinition of the function _Z4endlR6stream (once as llvm.func and once as func.func). However, it seems like this is not reproducible on Linux machines.

Start by running the provided cgeist command and FileCheck test with the inline stream::endl reproducer. Inspect the generated llvm.func for _Z4endlR6stream and compare the Linux and Mac behavior; done means the verifier accepts the linkage and no duplicate llvm.func/func.func definition remains.

Written by the indexing model from the issue text.