[clang-tools-extra] [clang-doc] Improve performance by adding a short circuit (PR #96809)

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PeterChou1 wrote:

> Second, this requires some more significant testing than is currently present for clang-doc. Additionally, we'd need to evaluate such an invasive change on multiple projects, and ensure that the documentation output has not changed due to this patch. Even then, I'd wouldn't be confident that we'd gotten it correct until tests were in place, and we could examine the differences with, and without this patch.

I agreed I'm working on a patch adding more e2e test to better evaluate the output, I'm comparing the current output of the llvm doc with the previous

https://github.com/llvm/llvm-project/pull/96809