Restore the ability to check if LLVMCreateObjectFile was successful

Summary:
Until r216870 LLVMCreateObjectFile returned nullptr in case of an error,
so callers could check if the call was successful. Now, it always
returns an OwningBinary wrapped as an LLVMObjectFileRef, so callers
can't check if the call was successul.

This results in a segfault running e.g.

 llvm-c-test --object-list-sections < /dev/null

So the old behaviour should be restored.

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5143

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217279 91177308-0d34-0410-b5e6-96231b3b80d8
Fix pr20078.

When linking llvm.global_ctors with the optional third element we have to handle
it specially and only copy the elements whose keys were also copied.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217281 91177308-0d34-0410-b5e6-96231b3b80d8