Qualify one more make_unique call.

The previous patch had effect, but missed this one. It seems MSVC
gets ADL-confused by the calls where the first argument is a function call?

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222968 91177308-0d34-0410-b5e6-96231b3b80d8