[Object] Return a symbol_iterator, rather than a basic_symbol_iterator, from

MachOObjectFile::getSymbolByIndex.

ObjectFile derivatives should prefer symbol_iterator/SymbolRef over
basic_symbol_iterator/BasicSymbolRef where possible, as the former
retain their link to the ObjectFile (rather than a SymbolicFile) and provide
more functionality.

No test for this: Existing code is working, and we don't have (m)any libObject
unit tests. I'll think about how we can test more systematically going forward.

llvm-svn: 351128