[lld][WebAssembly] Keep data in `foo` segments alive if `__start_foo` is referenced

[sbc100]

Summary

In emscripten we use named segments and export __start/__stop symbols
to extract data after the linker has run. In some cases that data is
not otherwise alive. For example:

https://github.com/emscripten-core/emscripten/pull/17854

The ELF linker already does this but is currently behind the `-z
nostart-stop-gc` flag. We could do that here to, but I'm not sure its
necessary.

[sbc100]

Still need to write a test for this..

lld/wasm/MarkLive.cpp

 
 #include "MarkLive.h"
 #include "Config.h"
 #include "InputChunks.h"
 #include "InputElement.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
 
+#include "lld/Common/CommonLinkerContext.h"
+#include "lld/Common/Strings.h"
+
 #define DEBUG_TYPE "lld"
 
 using namespace llvm;
 using namespace llvm::wasm;
 
 namespace lld {
 namespace wasm {
 
void MarkLive::run() {
   for (Symbol *sym : symtab->symbols())
     if (sym->isNoStrip() || sym->isExported())
       enqueue(sym);
 
   if (WasmSym::callDtors)
     enqueue(WasmSym::callDtors);
 
   // Enqueue constructors in objects explicitly live from the command-line.
-  for (const ObjFile *obj : symtab->objectFiles)
-    if (obj->isLive())
+  for (const ObjFile *obj : symtab->objectFiles) {
+    if (obj->isLive()) {
       enqueueInitFunctions(obj);
 
+      // If the `__start_` symbol for a given segment is referenced then keep
+      // all of its contents alive.
+      for (InputChunk *seg : obj->segments) {
+        if (isValidCIdentifier(seg->name)) {
+          StringRef name = saver().save("__start_" + seg->name);
+          if (symtab->find(name) || config->exportedSymbols.count(name) != 0) {
+            seg->live = true;
+          }
+        }
+      }
+    }
+  }
+
   mark();
 
   // If we have any non-discarded init functions, mark `__wasm_call_ctors` as
   // live so that we assign it an index and call it.
   if (isCallCtorsLive())
     WasmSym::callCtors->markLive();
 }