Fix to work with style of new YAML writer

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@171897 91177308-0d34-0410-b5e6-96231b3b80d8
Rename YAML key 'fixups:' to 'references:'

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@171898 91177308-0d34-0410-b5e6-96231b3b80d8