Revert "[NFC] [C++20] [Modules] Adjust the implementation of wasDeclEmitted to make it more clear"

This reverts commit 4399f2a.
This fails with Modules/aarch64-sme-keywords.cppm

Author: ChuanqiXu9

clang/lib/Serialization/ASTWriter.cpp

@@ -5704,12 +5704,6 @@ void ASTWriter::WriteDeclAndTypes(ASTContext &Context) {
     if (D->isFromASTFile())
       continue;
 
-    // Skip writing implicit declarations not owning by the current module.
-    // See the implementation of PrepareWritingSpecialDecls for example.
-    if (isWritingStdCXXNamedModules() && !D->getOwningModule() &&
-        D->isImplicit())
-      continue;
-
     // In reduced BMI, skip unreached declarations.
     if (!wasDeclEmitted(D))
       continue;
@@ -6288,7 +6282,8 @@ bool ASTWriter::wasDeclEmitted(const Decl *D) const {
     return true;
 
   bool Emitted = DeclIDs.contains(D);
-  assert((Emitted || GeneratingReducedBMI) &&
+  assert((Emitted || (!D->getOwningModule() && isWritingStdCXXNamedModules()) ||
+          GeneratingReducedBMI) &&
          "The declaration within modules can only be omitted in reduced BMI.");
   return Emitted;
 }