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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I’m not exactly sure how to test this change since this is not only platform-dependent but also path-dependent since we may end up producing absolute paths here.

I think this is a case where maybe we want to use unit tests. We have `clang/unittests/Basic/DiagnosticTest.cpp` or `FileManagerTest.cpp` already, so perhaps in one of those (depending on where the functionality ends up living)?

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