Add heading and remove leftover personal email addresses

clang's attribute support is mature by now so replace Sean's warning and email
address with a standard LLVM copyright heading.

Also remove a personal email address and credit docstring from CGObjCGNU that
shouldn't have been there.

This is in line with the LLVM developer policy introduced in r45410.
Contributors can add themselves to CREDITS.txt while active module owners are
listed in CODE_OWNERS.TXT.