[lld] [lld][MachO] Prevent doubled N_SO when comp_dir and name absolute (PR #71608)

[Shoaib Meenai via llvm-commits]

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@@ -1522,13 +1522,17 @@ void ObjFile::registerEhFrames(Section &ehFrameSection) {
 }
 
 std::string ObjFile::sourceFile() const {
+  const char *unitName = compileUnit->getUnitDIE().getShortName();
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smeenai wrote:

I would have wondered why the logic was written this way if your commit hadn't mentioned that it was matching the check in libObject :) Could you add a code comment here pointing to that logic so that it's easy to understand why it's written this way?

https://github.com/llvm/llvm-project/pull/71608