[clang] [DependencyScanningFilesystem] Make sure the local/shared cache filename lookups use only absolute paths (PR #66122)

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 commented:

Nice, this seems like an important aspect to get right. I left a couple of comments/questions.

https://github.com/llvm/llvm-project/pull/66122