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

[Peter Smith via llvm-commits]

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@@ -201,45 +238,126 @@ void MarkLive<ELFT>::enqueue(InputSectionBase *sec, uint64_t offset) {
     return;
   sec->partition = sec->partition ? 1 : partition;
 
+  if (TrackWhyLive) {
+    if (sym) {
+      // If a specific symbol is referenced, that makes it alive. It may in turn
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smithp35 wrote:

It looks like if the symbol is reference it will make its section live. The comment says "It may". Is that intentional?

Strangely I've never thought of it as alive, just live. I guess alive makes more sense in the sense of dead code.

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