[clang] [RFC] [clang][Toolchain] Treat "pc"/"unknown" vendor interchangeable (PR #97802)

[Azat Khuzhin via cfe-commits]

azat wrote:

I've excluded new test for windows (https://buildkite.com/llvm-project/github-pull-requests/builds/78458)

https://github.com/llvm/llvm-project/pull/97802