[PECOFF] Replace DLLNameAtom with COFFStringAtom.

DLLNameAtom is an atom whose content is a string. IdataAtom is not going to
be the only place we need such atom, so I want to generalize it.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197137 91177308-0d34-0410-b5e6-96231b3b80d8