[clang] [Clang][NFC] Clarify some SourceManager related code (PR #153527)

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon approved this pull request.

as someone dealing with c/c++ I don't really get why 0 column number is invalid, but that is already clearer, thanks

https://github.com/llvm/llvm-project/pull/153527