[clang-scan-deps] `DependencyScanningFilesystem` caches relative filename lookups

[akyrtzi]

A relative path would be used as a key for cache lookup and if the same relative path was used from another compiler invocation with a different working directory then the first cache entry was erroneously returned.

[akyrtzi]

Fixed on main via #66122

[akyrtzi]

/cherry-pick 36b37c7

[llvmbot]

/branch llvm/llvm-project-release-prs/issue67510

[llvmbot]

/pull-request llvm/llvm-project-release-prs#710