[lld] [lld][ELF] Demote symbols in discarded sections to Undefined. (PR #68777)

[Bevin Hansson via llvm-commits]

================
@@ -714,6 +714,22 @@ template <class ELFT> void Writer<ELFT>::copyLocalSymbols() {
       // No reason to keep local undefined symbol in symtab.
       if (!dr)
         continue;
+
+      // Demote locals which did not end up in any partition. This is similar
+      // to what we do in Driver.cpp, but that only works on globals.
+      // Don't do this for section symbols; they are expected to be lost.
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bevin-hansson wrote:

I'm a bit unsure about this. I had to prevent this for section symbols, because a few tests fail otherwise, mostly ones related to arm exidx sections. I don't understand the mechanisms at play enough to know exactly what is supposed to happen.

https://github.com/llvm/llvm-project/pull/68777