[clang-tools-extra] Fix Default Asset File locator for clang docs (PR #97505)

[Joshua Batista via cfe-commits]

bob80905 wrote:

Updated the description.
In my tests, there are at least 5 copies of this file in the overall build output directory:
```
<llvm-project-dir>/<MS-Build-Dir>/Debug/share/clang-doc
<llvm-project-dir>/<MS-Build-Dir>/Release/share/clang-doc
<llvm-project-dir>/<MS-Build-Dir>/MinSizeRel/share/clang-doc
<llvm-project-dir>/<MS-Build-Dir>/RelWithDebInfo/share/clang-doc
<llvm-project-dir>/<MS-Build-Dir>/share/clang-doc
```
Yes, one per configuration, and one underneath the binary directory.
I am unsure if we still need the copy in the binary directory, because I'm not sure which executable is run by default when running tests. 

https://github.com/llvm/llvm-project/pull/97505