Add a SymbolicFile interface between Binary and ObjectFile.

This interface allows IRObjectFile to be implemented without having dummy
methods for all section and segment related methods.

Both llvm-ar and llvm-nm are changed to use it. Unfortunately the mangler is
still not plugged in since it requires some refactoring to make a Module hold
a DataLayout.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201881 91177308-0d34-0410-b5e6-96231b3b80d8