[clang-tools-extra] [clang-doc][cmake] Copy assets to build directory (PR #95187)

[via cfe-commits]

PeterChou1 wrote:

> @petrhosek I'm pretty sure there's a better way to spell this, but I think we need something similar to properly test clang-doc w/o an install step.
> 
> cc: @PeterChou1

It looks like copy_directory_if_different is not available on windows

https://github.com/llvm/llvm-project/pull/95187