Remove Elf_Sym_Iter.

It was a fairly broken concept for an ELF only class.

An ELF file can have two symbol tables, but they have exactly the same
format. There is no concept of a dynamic or a static symbol. Storing this
on the iterator also makes us do more work per symbol than necessary. To fetch
a name we would:

* Find if we had a static or a dynamic symbol.
* Look at the corresponding symbol table and find the string table section.
* Look at the string table section to fetch its contents.
* Compute the name as a substring of the string table.

All but the last step can be done per symbol table instead of per symbol. This
is a step in that direction.

llvm-svn: 240939