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

[Ben Langmuir via cfe-commits]

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

Thanks for iterating! I find the current implementation much clearer.

The only thing I might quibble about is the "child" vs. "parent" terminology you changed: I think it's fairly ambiguous either way, because the node is the "child" from the perspective of a top-down include hierarchy, but it's the "parent" from the perspective of the bottom-up search.  You could maybe change it to `IncludedFile` or something, but I don't feel very strongly about it. Child is no worse than parent so if you prefer child I don't think you need to change it.

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