Comment parsing: when comment ranges are deserialized from multiple modules,
correctly order comments in SourceManager::isBeforeInTranslationUnit() order

Unfortunately, this is not as simple as it was implemented previously, and
actually requires doing a merge sort.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204936 91177308-0d34-0410-b5e6-96231b3b80d8