Clean up SmallString a bit

Move a common utility (assign(iter, iter)) into SmallVector (some of the
others could be moved there too, but this one seemed particularly
generic) and replace repetitions overrides with using directives.

And simplify SmallVector::assign(num, element) while I'm here rather
than thrashing these files (that cause everyone to rebuild) again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203374 91177308-0d34-0410-b5e6-96231b3b80d8