[ELF] Make isExported accurate early

LTO compilation might define symbols not in the symbol table (e.g.
__emutls_v.x in test/ELF/lto/wrap-unreferenced-before-codegen.test).
These symbols have a false `isExported` until
`demoteSymbolsAndComputeIsPreemptible`. This is usually benign as we do
not reference `isExported` that early.

Ensure that `isExported` is correct early. This helps remove a redundant
`isExported` computation in `demoteSymbolsAndComputeIsPreemptible`.

Author: MaskRay

lld/ELF/Driver.cpp

@@ -2157,9 +2157,12 @@ static void excludeLibs(Ctx &ctx, opt::InputArgList &args) {
     ArrayRef<Symbol *> symbols = file->getSymbols();
     if (isa<ELFFileBase>(file))
       symbols = cast<ELFFileBase>(file)->getGlobalSymbols();
-    for (Symbol *sym : symbols)
-      if (!sym->isUndefined() && sym->file == file)
+    for (Symbol *sym : symbols) {
+      if (!sym->isUndefined() && sym->file == file) {
         sym->versionId = VER_NDX_LOCAL;
+        sym->isExported = false;
+      }
+    }
   };
 
   for (ELFFileBase *file : ctx.objectFiles)
@@ -2545,11 +2548,17 @@ void LinkerDriver::compileBitcodeFiles(bool skipLinkedOutput) {
     auto *obj = cast<ObjFile<ELFT>>(file.get());
     obj->parse(/*ignoreComdats=*/true);
 
-    // Parse '@' in symbol names for non-relocatable output.
+    // For defined symbols in non-relocatable output,
+    // compute isExported and parse '@'.
     if (!ctx.arg.relocatable)
-      for (Symbol *sym : obj->getGlobalSymbols())
+      for (Symbol *sym : obj->getGlobalSymbols()) {
+        if (!sym->isDefined())
+          continue;
+        if (sym->includeInDynsym(ctx))
+          sym->isExported = true;
         if (sym->hasVersionSuffix)
           sym->parseSymbolVersion(ctx);
+      }
     ctx.objectFiles.push_back(obj);
   }
 }
@@ -3061,7 +3070,7 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
 
   // Handle --exclude-libs again because lto.tmp may reference additional
   // libcalls symbols defined in an excluded archive. This may override
-  // versionId set by scanVersionScript().
+  // versionId set by scanVersionScript() and isExported.
   if (args.hasArg(OPT_exclude_libs))
     excludeLibs(ctx, args);