[PECOFF][Writer] Define COFFBaseDefinedAtom as the base class of COFF defined atoms.

In order to support linking against DLL, the linker needs to create defined
atoms for jump tables and etc. Because such atoms are not read from a file,
they lack some information such as an ordinal. With this patch, COFFDefinedAtom
is split into two classes; one is the base class of all COFF defined atoms, and
another is a concrete class for atoms read from file. More classes inheriting
COFFBaseDefinedAtom will be added for jump tables and etc.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@185195 91177308-0d34-0410-b5e6-96231b3b80d8