[clang] [analyzer] Use getFileName and do not use realpath names (PR #126039)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov approved this pull request.

Could you add a description mentioning that real paths resolves symlinks and makes the tests fail when some forms of content-addressable filesystem is used (our internal environment)?

Otherwise LG!

https://github.com/llvm/llvm-project/pull/126039