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

[Paul Kirth via cfe-commits]

ilovepi wrote:

Oh, can you also update the title and commit message? You're not really adding a short circuit, you're just memoizing visited items, and avoiding reprocessing them.

A bit more context, and a summary of your findings on the changes to output would also be good to have in the commit log.

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