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

[Fangrui Song via llvm-commits]

================
@@ -119,17 +142,29 @@ void MarkLive<ELFT>::resolveReloc(InputSectionBase &sec, RelTy &rel,
     // group/SHF_LINK_ORDER rules (b) if the associated text section should be
     // discarded, marking the LSDA will unnecessarily retain the text section.
     if (!(fromFDE && ((relSec->flags & (SHF_EXECINSTR | SHF_LINK_ORDER)) ||
-                      relSec->nextInSectionGroup)))
-      enqueue(relSec, offset);
+                      relSec->nextInSectionGroup))) {
+      Symbol *canonicalSym = d;
+      if (TrackWhyLive && d->isSection()) {
+        if (Symbol *s = relSec->getEnclosingSymbol(offset))
----------------
MaskRay wrote:

As mentioned, getEnclosingSymbol iterates all symbols in the file, which is very slow. This perhaps should only be run after we get a path from a root to the --why-live specified symbol and know that the information is needed.

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