[WebAssembly][lld] Don't mark a file live from an undefine symbol

[dschuff]

Summary

Live symbols should only cause the files in which they are defined
to become live.
Fixes PR48156

For now this is only tested in emscripten: we're continuing
to work on reducing the test case further for an lld-style
unit test.

[sbc100]

lgtm .. assuming we do eventually figure out how to trigger this using a unit test. I generally agree with you that it rather strange that ::markLive is called at all on an undefined synbol.

[dschuff]

This landed as https://github.com/llvm/llvm-project/commit/cb77e877f8132b885fcac8b7532c58072537b9ed

lld/wasm/Symbols.cpp

if (auto *e = dyn_cast<DefinedEvent>(this))
     return e->event->live;
   if (InputChunk *c = getChunk())
     return c->live;
   return referenced;
 }
 
 void Symbol::markLive() {
   assert(!isDiscarded());
-  if (file != NULL)
+  if (file != NULL && isDefined())
     file->markLive();
   if (auto *g = dyn_cast<DefinedGlobal>(this))
     g->global->live = true;
   if (auto *e = dyn_cast<DefinedEvent>(this))
     e->event->live = true;
   if (InputChunk *c = getChunk())
     c->live = true;
   referenced = true;