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

[Fangrui Song via llvm-commits]

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@@ -93,14 +112,18 @@ static uint64_t getAddend(Ctx &, InputSectionBase &sec,
   return rel.r_addend;
 }
 
-template <class ELFT>
+template <class ELFT, bool TrackWhyLive>
 template <class RelTy>
-void MarkLive<ELFT>::resolveReloc(InputSectionBase &sec, RelTy &rel,
-                                  bool fromFDE) {
+void MarkLive<ELFT, TrackWhyLive>::resolveReloc(InputSectionBase &sec,
+                                                RelTy &rel, bool fromFDE) {
   // If a symbol is referenced in a live section, it is used.
   Symbol &sym = sec.file->getRelocTargetSym(rel);
   sym.used = true;
 
+  LiveReason reason;
+  if (TrackWhyLive)
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MaskRay wrote:

`if constexpr `

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