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

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SergejSalnikov wrote:

This PR is functionally no-op. It's purely code optimisation do reduce the number of back-to-back  getSLocEntry and getFileID calls.

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