[clang] [clang-tools-extra] [Clang] [Diagnostics] Simplify filenames that contain '..' (PR #143520)

[Justin Cady via cfe-commits]

justincady wrote:

Instead of operating in place, could you avoid the network mount entirely by creating the required structure under /tmp and running the test there?

https://github.com/llvm/llvm-project/pull/143520