[llvm] [Linker] Do not keep a private member of a non-prevailing comdat group (PR #69143)

[Fangrui Song via llvm-commits]

MaskRay wrote:

```
> cat foo.h
inline int foo(int a) { return a + 1; }
int bar(int a);
```

Remove `int bar(int a);`


https://github.com/llvm/llvm-project/pull/69143