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

[via cfe-commits]

Sirraide wrote:

> should the header filter apply to the original filename,

I mean, I guess this beacuse it’s what the user specified and it’s what we’re currently doing? It’s just that the end result might be weird, e.g. if a user writes `-exclude-header=filter='a/foo.h'` and then we print diagnostics in `'a/foo.h'` because it was actually included via `"a/b/../foo.h"` (assuming I’m not misinterpreting what this option does), but maybe that’s ok?

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