[PECOFF] Read files lazily.

ExportedSymbolRenameFile is not always used. In most cases we don't
need to read given files at all. So lazy load would help. This doesn't
change the meaining of the program.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@208818 91177308-0d34-0410-b5e6-96231b3b80d8