[clang] [clang][modules] Move `SLocEntry` search into `ASTReader` (PR #66966)

[Ben Langmuir via cfe-commits]

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

LGTM; you might need a `std::move` in `readSLocOffset` to appease an older compiler.

https://github.com/llvm/llvm-project/pull/66966