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

[via cfe-commits]

PeterChou1 wrote:

> We already have set of CMake variables that we use to refer to `bin` or `lib` directory which account for various configurations:
> 
> https://github.com/llvm/llvm-project/blob/c02e8f762a410e55581866c43636efcd6504c1bd/llvm/CMakeLists.txt#L446-L467
> 
> I think we should introduce another one for the `share` directory and use that.

I've introduced a variable in the root cmakelists.txt I'm not very familiar with cmake though I'm sure if this is the correct approach

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