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

[Daniel Thornburgh via llvm-commits]

================
@@ -42,16 +44,30 @@ using namespace lld;
 using namespace lld::elf;
 
 namespace {
-template <class ELFT> class MarkLive {
+
----------------
mysterymath wrote:

Done.

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