[clang] [C++20] [Modules] Don't update MarkAsUsed information for decls from named modules (PR #174687)

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``````````bash
git-clang-format --diff origin/main HEAD --extensions cppm,cpp -- clang/test/Modules/class-instantiate-no-change.cppm clang/lib/Serialization/ASTWriter.cpp --diff_from_common_commit
``````````

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View the diff from clang-format here.
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``````````diff
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index a211e9866..a6d85600f 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -7769,8 +7769,10 @@ void ASTWriter::DeclarationMarkedUsed(const Decl *D) {
   if (Chain && Chain->isProcessingUpdateRecords()) return;
   assert(!WritingAST && "Already writing the AST!");
 
-  // If the declaration comes from module unit, it is meaningless to mark it as used.
-  if (auto *M = D->getOwningModule(); M && M->getTopLevelModule()->isNamedModule())
+  // If the declaration comes from module unit, it is meaningless to mark it as
+  // used.
+  if (auto *M = D->getOwningModule();
+      M && M->getTopLevelModule()->isNamedModule())
     return;
 
   // If there is *any* declaration of the entity that's not from an AST file,

``````````

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https://github.com/llvm/llvm-project/pull/174687