[clang-doc] fix broken tests (#100260)

Summary:
This patch fixes the broken test caused by clang-doc which was caused by
#100251

https://lab.llvm.org/buildbot/#/builders/144/builds/3080

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Differential Revision: https://phabricator.intern.facebook.com/D60251218

Author: PeterChou1

clang-tools-extra/clang-doc/HTMLGenerator.cpp

@@ -983,7 +983,7 @@ static llvm::Error serializeIndex(ClangDocContext &CDCtx) {
   llvm::json::OStream J(OS, 2);
   std::function<void(Index)> IndexToJSON = [&](const Index &I) {
     J.object([&] {
-      //J.attribute("USR", toHex(llvm::toStringRef(I.USR)));
+      J.attribute("USR", toHex(llvm::toStringRef(I.USR)));
       J.attribute("Name", I.Name);
       J.attribute("RefType", getRefType(I.RefType));
       J.attribute("Path", I.getRelativeFilePath(""));