[dsymutil] Check AttrInfo.Name validity before using it

[loladiro]

Summary

This upstreams a patch from the osxcross [1] toolchain.
It appears that llvm-dsymutil crashes at this place during GCC
bootstrap. Adding the check here seems reasonable, since it operates
on arbitrary input DWARF, not necessarily generated by the LLVM
toolchain, and it seems the un-mangled name need not necessarily exist.

Patch by Thomas Pöchtrager

[1] https://github.com/tpoechtrager/osxcross

tools/dsymutil/DwarfLinker.cpp

if (AttrInfo.MangledName && AttrInfo.MangledName != AttrInfo.Name)
       Unit.addNameAccelerator(Die, AttrInfo.MangledName,
                               AttrInfo.MangledNameOffset,
                               Tag == dwarf::DW_TAG_inlined_subroutine);
     if (AttrInfo.Name)
       Unit.addNameAccelerator(Die, AttrInfo.Name, AttrInfo.NameOffset,
                               Tag == dwarf::DW_TAG_inlined_subroutine);
   } else if (isTypeTag(Tag) && !AttrInfo.IsDeclaration &&
              getDIENames(InputDIE, AttrInfo)) {
+    if (AttrInfo.Name)
       Unit.addTypeAccelerator(Die, AttrInfo.Name, AttrInfo.NameOffset);
   }
 
   // Determine whether there are any children that we want to keep.
   bool HasChildren = false;
   for (auto Child: InputDIE.children()) {
     unsigned Idx = U.getDIEIndex(Child);
     if (Unit.getInfo(Idx).Keep) {
       HasChildren = true;