[clang-tools-extra] [clang-tidy] Fix loading clang-tidy as a Clang frontend plugin (PR #221961)

[Baranov Victor via cfe-commits]

vbvictor wrote:

Can we write in tests/docs that windows in not supported. By `// REQUIRES: clang-plugin` its not clean that we don't have plugin because it was never build (at least to me)


https://github.com/llvm/llvm-project/pull/221961