Fix CodeMap issue in expand_include_str()
auto merge of #11401 : michaelwoerister/rust/issue11322, r=alexcrichton

`expand_include_str()` in libsyntax seems to have corrupted the CodeMap by always setting the BytePos of any included files to zero. It now uses `CodeMap::new_filemap()` which should set everything properly. This should fix issue #11322 but I don't want to close it before I have confirmation from the reporters that the problem is indeed fixed.