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

[Justin Cady via cfe-commits]

justincady wrote:

@Sirraide I'm seeing failures in the newly added test cases with a repo on a network mount. I see the code changes handle that scenario, but if I understand correctly the tests currently do not.

Could the tests be updated to either be skipped or validate that the canonicalization does _not_ happen in that case?

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