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

[Peter Smith via llvm-commits]

================
@@ -559,6 +559,12 @@ defm wrap : Eq<"wrap", "Redirect symbol references to __wrap_symbol and "
                        "__real_symbol references to symbol">,
             MetaVarName<"<symbol>">;
 
+defm why_live
----------------
smithp35 wrote:

I think this will need adding to docs/ld.lld.1 too.

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