[dsymutil] Pass LinkOptions by value instead of const ref.

When looping over the difference architectures in a fat binary, we
modify the link options before dispatching the link step to a different
thread. Passing the options by cont reference is not thread safe, as we
might modify its fields before the whole sturct is copied over.

Given that the link options are already stored in the DwarfLinker, we
can easily fix this by passing a copy of the link options instead of a
reference, which would just get copied later on.

llvm-svn: 367348