Tidy up a bit of r304516, use SmallVector::assign rather than for loop

This might give a few better opportunities to optimize these to memcpy
rather than loops - also a few minor cleanups (StringRef-izing,
templating (to avoid std::function indirection), etc).

The SmallVector::assign(iter, iter) could be improved with the use of
SFINAE, but the (iter, iter) ctor and append(iter, iter) need it to and
don't have it - so, workaround it for now rather than bothering with the
added complexity.

(also, as noted in the added FIXME, these assign ops could potentially
be optimized better at least for non-trivially-copyable types)

llvm-svn: 304566