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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> My expectation would be that if I specify a header filter I'm not going to use weird paths like a/b/../foo.h, but just a/foo.h because that is where foo.h lives.

What about symlinks though? Would you expect that passing `path/to/file` fails because `path` is a symlink and you should have specified `/var/foo/bar/to/file`? (It's basically the same problem -- do we make the user pass the resolved path or do we canonicalize the path for the user and use that to do the filtering?)

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