[PECOFF] Rename IdataPassFile and move it to Atoms.h.

I'm planning to create a new pass for the DLL export table, and I want to use
the class both from IdataPass and the new pass, EdataPass. So move the class to
a common place.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197132 91177308-0d34-0410-b5e6-96231b3b80d8