Introduce convenience typedefs for the 4 ELF object types.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181509 91177308-0d34-0410-b5e6-96231b3b80d8
libDebugInfo depends on libObject nowadays.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181510 91177308-0d34-0410-b5e6-96231b3b80d8