Restore functionality to clang-doc asset locator in multi-config builds

[bob80905]

Multi-config builds fail to build the clang-doc project, because two asset files aren't found, when the executable running the test runs under the Debug config.
This is because the asset locator expects that the executable running the test is located at the top level build directory, not under an extra directory (e.g., Debug).
AC: fix the asset locator so that the project builds / passes in any configuration.

[bob80905]

Closed by #98099