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

[Justin Bogner via cfe-commits]

bogner 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.

We shouldn't need the one in the binary directory - IIUC one per configuration will be sufficient, and the one that's never used is potentially confusing.

Please also test locally that the install target does the right thing with these changes. You can use `-DCMAKE_INSTALL_PREFIX=/some/temp/dir` to make sure it doesn't install to some system location and is easy to spot check.

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