[libc] Update libc namespace clang-tidy checks

Namespace macro that should be used to declare a new namespace is
updated from LIBC_NAMESPACE to LIBC_NAMESPACE_DECL which by default has
hidden visibility (#97109). This commit updates the clang-tidy checks to
match the new policy.

clang-tools-extra/clang-tidy/llvmlibc/ImplementationInNamespaceCheck.cpp

@@ -33,17 +33,17 @@ void ImplementationInNamespaceCheck::check(
   if (NS == nullptr || NS->isAnonymousNamespace()) {
     diag(MatchedDecl->getLocation(),
          "declaration must be enclosed within the '%0' namespace")
-        << RequiredNamespaceMacroName;
+        << RequiredNamespaceHiddenMacroName;
     return;
   }
   if (Result.SourceManager->isMacroBodyExpansion(NS->getLocation()) == false) {
     diag(NS->getLocation(), "the outermost namespace should be the '%0' macro")
-        << RequiredNamespaceMacroName;
+        << RequiredNamespaceHiddenMacroName;
     return;
   }
   if (NS->getName().starts_with(RequiredNamespaceStart) == false) {
     diag(NS->getLocation(), "the '%0' macro should start with '%1'")
-        << RequiredNamespaceMacroName << RequiredNamespaceStart;
+        << RequiredNamespaceHiddenMacroName << RequiredNamespaceStart;
     return;
   }
 }

clang-tools-extra/clang-tidy/llvmlibc/NamespaceConstants.h

@@ -11,6 +11,8 @@
 namespace clang::tidy::llvm_libc {
 
 const static llvm::StringRef RequiredNamespaceStart = "__llvm_libc";
+const static llvm::StringRef RequiredNamespaceHiddenMacroName =
+    "LIBC_NAMESPACE_DECL";
 const static llvm::StringRef RequiredNamespaceMacroName = "LIBC_NAMESPACE";
 
 } // namespace clang::tidy::llvm_libc