[lld] [LTO][ELF][lld] Use unique string saver in ELF bitcode symbol parsing (PR #106670)

[Teresa Johnson via llvm-commits]

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@@ -56,6 +57,10 @@ bool hasContext();
 
 inline llvm::StringSaver &saver() { return context().saver; }
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teresajohnson wrote:

Does it make sense to just switch this one to UniqueStringSaver? That would automatically provide the same benefit to other clients like COFF and MachO

https://github.com/llvm/llvm-project/pull/106670