[PECOFF] Split LocallyImportedSymbolFile into two classes.

I have a plan to use VirtualArchiveFile in this file in the near future.
This change should improve the readability by itself, too.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@208365 91177308-0d34-0410-b5e6-96231b3b80d8