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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I think the new plan forward makes sense. But also: thank you to everyone on this thread for the excellent collaboration on identifying an issue, getting the previous incarnation reverted, and discussing a good path forward. :-)

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