In SourceManager::isBeforeInTranslationUnit, if we are trying to compare two...

In SourceManager::isBeforeInTranslationUnit, if we are trying to compare two source locations with no common ancestor in the include stack, determine order by assuming memory buffers preceed files, and then that FileIDs are created in order.

The later assumption is patently false, but this was already broken -- this situation is conceptually impossible, my feeling is we should fix SourceManager and friends to make it impossible in practice as well. However, we need to fix PR5662 and perhaps some other things involving memory buffers first. In the short term I'm pretty sure this is reliable.

Chris, Argiris, is this going to break anything that wasn't already broken?

llvm-svn: 90280