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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> We could try that (assuming that `/tmp` exists on every system but I sure hope so); it might run into issues if we somehow pick a file/directory name that something else is already using, but that’s probably not too likely.

Windows doesn't have `/tmp` for example. I don't think we have any [substitutions](https://llvm.org/docs/CommandGuide/lit.html#substitutions) for getting the temp directory.

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