[lld] [lld][MachO] Support for -interposable (PR #131813)

[John Holdsworth via llvm-commits]

johnno1962 wrote:

I'm sensing a reluctance to take my word for Apple's linker applying -interposable for symbols inside executables. To close this out how about I go with @smeenai's original suggestion in the mentioned issue as below, update the test and not implement this. With Xcode 16 an app's "main image" is a .dylib now anyway so it's not important to me to fight this battle.
```
  bool interposable = (config->namespaceKind == NamespaceKind::flat || config->interposable) &&
                      config->outputType != MachO::MH_EXECUTE &&
                      !isPrivateExtern;

https://github.com/llvm/llvm-project/pull/131813