[clang-tools-extra] [clang-doc] fix bug introduced by asset test (PR #97540)

[via cfe-commits]

https://github.com/dyung requested changes to this pull request.

I believe (and the pre-merge testing seems to validate my belief) that this breaks the case where you build with ninja + Visual Studio. Also, I believe the Xcode generator is similar to the Visual Studio generator in that they both are multi-config generators, so that configuration is likely failing as well.

https://github.com/llvm/llvm-project/pull/97540