RawComments are sorted by comparing underlying SourceLocation's. This is done by comparing FileID and Offset; when the FileID is the same it means
the locations are within the same TU and the Offset is used, etc.
FileID, from the source code: "A mostly-opaque identifier, where 0 is "invalid", >0 is this module, and <-1 is something loaded from another
module.". That said, when de-serializing SourceLocations, FileID's from RawComments loaded from other modules get negative IDs where previously they
were positive. This makes imported RawComments unsorted, leading to a wrong merge with other comments from the current TU. Fix that by sorting RawComments
properly after de-serialization and before merge.
This fixes an assertion in ASTContext::getRawCommentForDeclNoCache, which fires only in a debug build of clang. There's seems to be no reliable way to test this.
Additionally, I tried to use llvm::array_pod_sort, but that didn't seem to cope well with BeforeThanCompare<RawComment>(SourceMgr) or lambdas, and a SourceMgr is needed to perform the sort.