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

[Peter Smith via llvm-commits]

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@@ -42,16 +44,30 @@ using namespace lld;
 using namespace lld::elf;
 
 namespace {
-template <class ELFT> class MarkLive {
+
+using SecOffset = std::pair<InputSectionBase *, unsigned>;
+
+// Something that can have an independent reason for being live.
+using LiveObject = std::variant<InputSectionBase *, Symbol *, SecOffset>;
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smithp35 wrote:

In the context of a linker, using object and obj could be easily thought of as a relocatable object.

Perhaps `LiveItem` or `LiveEntity` instead? With item or entity in LiveReason below. 

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