Use pointers to iterate over symbols.

While here, don't report a dummy symbol for relocations that don't have symbols.
We used to says such relocations were for the first defined symbol, but now we
return end_symbols(). The llvm-readobj output change agrees with otool.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180214 91177308-0d34-0410-b5e6-96231b3b80d8