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

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

Turns out some clients are calling `isBeforeInTranslationUnit()` before checking if both `SourceLocations` are indeed in the same TU. I left behind a FIXME to call `llvm_unreachable()`, but for now, I just compare the `FileIDs` to keep things working.

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