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

[Peter Smith via llvm-commits]

================
@@ -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>;
+
+// The most proximate reason that an object is live.
+struct LiveReason {
+  std::optional<LiveObject> obj;
+  StringRef desc;
+};
+
----------------
smithp35 wrote:

It looks like only a subset of the functions use TrackWhyLive (3 at rough count), just in case we're trying to minimise code-size, we could split the non parameter using functions into a base class. It may not be worth it for just (ELFT * TrackWhyLive) instantiations though.

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