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

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SergejSalnikov wrote:

As there was some push back from other members of the team regarding readability of this CL. I'll let the decision on whether to merge this PR to the llvm team. Please merge the CL on my behalf if you think it's worth having it in.

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