[Object] Split the ELF interface into 3 parts.

* ELFTypes.h contains template magic for defining types based on endianess, size, and alignment.
* ELFFile.h defines the ELFFile class which provides low level ELF specific access.
* ELFObjectFile.h contains ELFObjectFile which uses ELFFile to implement the ObjectFile interface.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188022 91177308-0d34-0410-b5e6-96231b3b80d8