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

[Joshua Batista via cfe-commits]

bob80905 wrote:

An important assumption I'm making here is that the Debug directory cannot exist without the Release directory. If this assumption doesn't hold, then this fix will fail if only the release directory exists and debug is missing. I believe by the nature of how the llvm project is generated, both directories are generated at all times.

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