r267296 used std::piecewise_construct without using
std::forward_as_tuple, and r267298 hacked it out (using an emplace_back
followed by a couple of reset() calls) because of a problem on a bot.
I'm finally circling back to call forward_as_tuple as I should have to
begin with (thanks to David Blaikie for pointing out the missing piece).
Note that this code uses emplace_back() instead of
push_back(make_pair()) because the move constructor for TrackingMDRef is
expensive (cheaper than a copy, but still expensive).
llvm-svn: 272306
c3f8997386