[PECOFF] Split COFFDefinedAtom to add COFFBSSAtom class.

The BSS atom is similar to the regular defined atom, but it's different
in the sense that it does not have contents. Until now we assumed all the
defined atoms have its contents. That did not fit well to the BSS atom.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@187453 91177308-0d34-0410-b5e6-96231b3b80d8