[PECOFF] Split ExportedSymbolRenameFile.

The code to manage resolvable symbols is now separated from
ExportedSymbolRenameFile so that other class can reuse it.
I'm planning to use it to find the entry function symbol
based on resolvable symbols.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@213322 91177308-0d34-0410-b5e6-96231b3b80d8