[lld] [lld][ELF] Add --why-live flag (inspired by Mach-O) (PR #127112)

[Fangrui Song via llvm-commits]

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@@ -42,6 +44,10 @@ using namespace lld;
 using namespace lld::elf;
 
 namespace {
+
+// Something that can be the most proximate reason that something else is alive.
+typedef std::variant<InputSectionBase *, Symbol *> LiveReason;
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MaskRay wrote:

prefer `using` for C++

https://github.com/llvm/llvm-project/pull/127112