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

[James Y Knight via cfe-commits]

jyknight wrote:

> the main thing you care about is being able to turn off the canonicalisation entirely with -no-canonical-prefix, is that right?

In terms of not breaking the Google buildsystem: yes. But if the question is whether the current patch would be fine if its behavior was also disabled by that flag: I'm not in favor of that.

While that _would_ address the breakage which brought this PR to my attention, my worry about ill-effects from canonicalization of the user's build-dir-relative file paths into absolute paths is a more general one.

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