[ELF] Remove redundant isExported computation

Commit 2a26292 made `isExported`
accurate except a few linker-synthesized symbols in finalizeSections.
We can collect these linker-synthesized symbols into a vector
and avoid recomputation for other symbols.

This is reland of 1a4d6de after
`isExported` has been made accurate by f10441a

Author: MaskRay

lld/ELF/Config.h

@@ -619,6 +619,7 @@ struct Ctx : CommonLinkerContext {
   };
   ElfSym sym{};
   std::unique_ptr<SymbolTable> symtab;
+  SmallVector<Symbol *, 0> synthesizedSymbols;
 
   SmallVector<std::unique_ptr<MemoryBuffer>> memoryBuffers;
   SmallVector<ELFFileBase *, 0> objectFiles;

lld/ELF/Writer.cpp

@@ -149,6 +149,7 @@ static Defined *addOptionalRegular(Ctx &ctx, StringRef name, SectionBase *sec,
   if (!s || s->isDefined() || s->isCommon())
     return nullptr;
 
+  ctx.synthesizedSymbols.push_back(s);
   s->resolve(ctx, Defined{ctx, ctx.internalFile, StringRef(), STB_GLOBAL,
                           stOther, STT_NOTYPE, val,
                           /*size=*/0, sec});
@@ -295,13 +296,13 @@ static void demoteSymbolsAndComputeIsPreemptible(Ctx &ctx) {
                   sym->type)
             .overwrite(*sym);
         sym->versionId = VER_NDX_GLOBAL;
+        if (hasDynsym && sym->includeInDynsym(ctx))
+          sym->isExported = true;
       }
     }
 
-    if (hasDynsym) {
-      sym->isExported = sym->includeInDynsym(ctx);
+    if (hasDynsym)
       sym->isPreemptible = sym->isExported && computeIsPreemptible(ctx, *sym);
-    }
   }
 }
 
@@ -1838,6 +1839,12 @@ template <class ELFT> void Writer<ELFT>::finalizeSections() {
     }
   }
 
+  // If the previous code block defines any non-hidden symbols (e.g.
+  // __global_pointer$), they may be exported.
+  if (ctx.hasDynsym)
+    for (Symbol *sym : ctx.synthesizedSymbols)
+      sym->isExported = sym->includeInDynsym(ctx);
+
   demoteSymbolsAndComputeIsPreemptible(ctx);
 
   if (ctx.arg.copyRelocs && ctx.arg.discard != DiscardPolicy::None)