Split Undefined and UndefinedElf.

I am working on adding LTO support to the new ELF lld.

In order to do that, it will be necessary to represent defined and
undefined symbols that are not from ELF files. One way to do it is to
change the symbol hierarchy to look like

Defined : SymbolBody
Undefined : SymbolBody

DefinedElf<ELFT> : Defined
UndefinedElf<ELFT> : Undefined

Another option would be to use bogus Elf_Sym, but I think that is
getting a bit too hackish.

This patch does the Undefined/UndefinedElf. Split. The next one
will do the Defined/DefinedElf split.

llvm-svn: 256289