Remove DefinedElf class.

DefinedElf was a superclass of DefinedRegular and SharedSymbol classes
and represented the notion of defined symbols created for ELF symbols.

It turned out that we didn't use that class often. We had only two
occurrences of dyn_cast'ing to DefinedElf, and both were easily
rewritten without it.

The class was also a bit confusing. The concept of "created for ELF
symbol" is orthogonal to defined/undefined types. However, we had
two distinct classes, DefinedElf and UndefinedElf.

This patch simply removes the class. Now the class hierarchy is one
level shallower.

llvm-svn: 265234