More MSVC bot appeasement. Explicitly define rvalue methods on SortKey.

OwningAtomPtr does not have OwningAtomPtr(OwningAtomPtr&) or the equivalent
operator= as we only want to use rvalue references in it.

SortKey didn't like this on MSVC as it was synthesizing SortKey(SortKey&) and
trying to use the OwningAtomPtr(OwningAtomPtr&) method which was private an
unimplemented.

Now we explicitly have the methods on SortKey so hopefully the bot will be
happier.

llvm-svn: 264077