[clang] [C++20][Modules] Improve namespace look-up performance for modules. (PR #171769)

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<!--LLVM CODE FORMAT COMMENT: {clang-format}-->


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You can test this locally with the following command:
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``````````bash
git-clang-format --diff origin/main HEAD --extensions cpp -- clang/lib/Serialization/ASTReader.cpp clang/lib/Serialization/ASTWriter.cpp --diff_from_common_commit
``````````

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<details>
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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 231e0a206..971f59913 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -4456,7 +4456,7 @@ public:
                "An external namespace decl that is not "
                "key declaration of this TU");
         Chain->forEachImportedKeyDecl(D, [&AddDecl](const Decl *D) {
-          AddDecl(cast<NamedDecl>(const_cast<Decl*>(D)));
+          AddDecl(cast<NamedDecl>(const_cast<Decl *>(D)));
         });
       } else {
         AddDecl(D);

``````````

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https://github.com/llvm/llvm-project/pull/171769