unique_ptrify IRObjectFile::createIRObjectFile

I took a guess at the changes to the gold plugin, because that doesn't
seem to build by default for me. Not sure what dependencies I might be
missing for that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217056 91177308-0d34-0410-b5e6-96231b3b80d8