Add file+span to hir::ExprNode, then trace back through the parser and such to populate them and forward into passes to make them propagate through transforms. Then we can make our error messages Actually Heckin' Work past the parser, instead of panicking uglily everywhere.
No error info should ever be needed past typeck, so the more invasive code transformations of TckPass shouldn't need to preserve that info.
Done in f75091a59067 . Not actually filled out or used much though. Continue work in #2.