reimplement the guts of SourceManager::isBeforeInTranslationUnit

to be algorithmically faster and avoid an std::map.  This routine
basically boils down to finding the nearest common ancestor in a
tree, and we (implicitly) have information about nesting depth,
use it!

This wraps up rdar://7948633 - SourceManager::isBeforeInTranslationUnit has poor performance

llvm-svn: 103239