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

[via cfe-commits]

Sirraide wrote:

Ok, looks like the clang-tidy test failure is related to the `-header-filter` option:
```C++
// Check that `-header-filter` operates on the same file paths as paths in
// diagnostics printed by ClangTidy.
#include "dir1/dir2/../header_alias.h"
// CHECK_HEADER_ALIAS: dir1/dir2/../header_alias.h:1:11: warning: single-argument constructors
```

So, I guess my question is now, should the header filter apply to the original filename, the simplified filename, or both?

@AaronBallman Thoughts? Or alternatively who do best ping for clang-tidy related questions?

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