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

The preivous implementation of wasDeclEmitted may be confusing that
why we need to filter the declaration not from modules. Now adjust the
implementations to avoid the problems.

Author: ChuanqiXu9

clang/lib/Serialization/ASTWriter.cpp

@@ -5704,6 +5704,12 @@ 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;
@@ -6282,8 +6288,7 @@ bool ASTWriter::wasDeclEmitted(const Decl *D) const {
     return true;
 
   bool Emitted = DeclIDs.contains(D);
-  assert((Emitted || (!D->getOwningModule() && isWritingStdCXXNamedModules()) ||
-          GeneratingReducedBMI) &&
+  assert((Emitted || GeneratingReducedBMI) &&
          "The declaration within modules can only be omitted in reduced BMI.");
   return Emitted;
 }