[clang] [clang][modules] Remove preloaded SLocEntries from PCM files (PR #66962)

[Ben Langmuir via cfe-commits]

benlangmuir wrote:

Thanks for laying it out. I also find this easier to understand in your latest changes because the comments in `isBeforeInTranslationUnit` lay out the intended ordering without relying on `isInTheSameTranslationUnit` returning `false` for builtin buffers, which is the part that seemed suspect.

https://github.com/llvm/llvm-project/pull/66962