[clang] [clang][unittests] Fix linker error for DirectoryWatcherTest with CLANG_LINK_CLANG_DYLIB (PR #178455)

[Aiden Grossman via cfe-commits]

https://github.com/boomanaiden154 commented:

> the test would fail with:

The description seems to be missing what the test is failing with?

The issue that this fixes should also be mentioned in the PR description. You also need to update your email address to be public so that the PR can land without using your Github noreply email.

https://github.com/llvm/llvm-project/pull/178455