[clang-tools-extra] [clang-doc] Add individual target for unit tests (PR #185695)

[Paul Kirth via cfe-commits]

https://github.com/ilovepi commented:

LGTM, but I don't see this pattern used elsewhere. @petrhosek is this OK? is there a better way to do this?

https://github.com/llvm/llvm-project/pull/185695