Object: Add ELF types to ELFType.

These types are defined in ELFFile, so in order to use them, you have
to write ELFFile<ELFT>::SomeType. But there seems to be no reason to have
ELFFile have these types. This patch allows you to write ELFT::SomeType
instead.

This simplifies libObject users.
This is an example: http://reviews.llvm.org/D18129

http://reviews.llvm.org/D18130

llvm-svn: 263504