[ELF] Fix copy relocation when two symbols share the same Symbol instance.

In glibc libc.so.6, the multiple versions of sys_errlist share the same Symbol instance. When sys_errlist is copy relocated, we would replace SharedSymbol with Defined in the first iteration of the following loop:

  for (SharedSymbol *Sym : getSymbolsAt<ELFT>(SS))

Then in the second iteration, we think the symbol (which has been changed to Defined) is still SharedSymbol and screw up (the address ends up in the `Size` field).

llvm-svn: 334432