[clang] [clang][docs] Fix docs-clang-html. (PR #142387)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

this is tripping on CI and my machine.

To repro I need to delete `$BUILD_DIR/tools/clang/docs` and build with `TZ=UTC ninja   docs-clang-man`

I find it necessary to delete the doc directory to trigger this.

https://github.com/llvm/llvm-project/pull/142387