Handle relocations that don't point to symbols.

In ELF (as in MachO), not all relocations point to symbols. Represent this
properly by using a symbol_iterator instead of a SymbolRef. Update llvm-readobj
ELF's dumper to handle relocatios without symbols.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183284 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Object/COFFObjectFile.cpp b/lib/Object/COFFObjectFile.cpp
index f5b49ab..bc5958d 100644
--- a/lib/Object/COFFObjectFile.cpp
+++ b/lib/Object/COFFObjectFile.cpp
@@ -712,13 +712,11 @@
   Res = toRel(Rel)->VirtualAddress;
   return object_error::success;
 }
-error_code COFFObjectFile::getRelocationSymbol(DataRefImpl Rel,
-                                               SymbolRef &Res) const {
+symbol_iterator COFFObjectFile::getRelocationSymbol(DataRefImpl Rel) const {
   const coff_relocation* R = toRel(Rel);
   DataRefImpl Symb;
   Symb.p = reinterpret_cast<uintptr_t>(SymbolTable + R->SymbolTableIndex);
-  Res = SymbolRef(Symb, this);
-  return object_error::success;
+  return symbol_iterator(SymbolRef(Symb, this));
 }
 error_code COFFObjectFile::getRelocationType(DataRefImpl Rel,
                                              uint64_t &Res) const {