IR: Use SmallVector instead of std::vector of TrackingMDRef

Don't use std::vector<TrackingMDRef>, since (at least in some versions
of libc++) std::vector apparently copies values on grow operations
instead of moving them.  Found this when I was temporarily deleting the
copy constructor for TrackingMDRef to investigate a performance
bottleneck.

llvm-svn: 266909