[clang] [clang] Optimize SourceManager.getSpellingLocSlowCase and SourceManager.getFileLocSlowCase (PR #164269)

[Aaron Ballman via cfe-commits]

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

Thanks! The performance measurements seem to be a very slight improvement, and the code isn't significantly less readable, so I'd say let's land the changes.

https://github.com/llvm/llvm-project/pull/164269